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A TREATISE
ON
ELECTRICITY AND MAGNETISM
MAXWELL
VOL. I.
Uonfron
MACMILLAN AND CO.
PUBLISHERS TO THE UNIVERSITY OF
Clarendon press Series
A TREATISE
ON
ELECTRICITY AND MAGNETISM
BY
JAMES CLERK MAXWELL, M.A
LLD. EDIN., P.E.SS. LONDON AND EDINBURGH
HONORARY FELLOW OF TRINITY COLLEGE,
AND PROFESSOR OF EXPERIMENTAL PHYSICS
IN THE UNIVERSITY OF CAMBRIDGE
VOL. I
AT THE CLARENDON PRESS
1873
[All rights reserved]
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PREFACE.
THE fact that certain bodies, after being rubbed,
appear to attract other bodies, was known to the
ancients. In modern times, a great variety of other
phenomena have been observed, and have been found
to be related to these phenomena of attraction. They
have been classed under the name of Electric phe
nomena, amber, ?i\eKTpov, having been the substance
in which they were first described.
Other bodies, particularly the loadstone, and pieces
of iron and steel which have been subjected to certain
processes, have also been long known to exhibit phe
nomena of action at a distance. These phenomena,
with others related to them, were found to differ from
the electric phenomena, and have been classed under
the name of Magnetic phenomena, the loadstone, /zayi^?,
being found in the Thessalian Magnesia.
These two classes of phenomena have since been
found to be related to each other, and the relations
between the various phenomena of both classes, so
far as they are known, constitute the science of Elec-
tromagnetism.
In the following Treatise I propose to describe the
812245
Ti PREFACE.
most important of these phenomena, to shew how they
may be subjected to measurement, and to trace the
mathematical connexions of the quantities measured.
Having thus obtained the data for a mathematical
theory of electromagnetism, and having shewn how
this theory may be applied to the calculation of phe
nomena, I shall endeavour to place in as clear a light
as I can the relations between the mathematical form
of this theory and that of the fundamental science of
Dynamics, in order that we may be in some degree
prepared to determine the kind of dynamical pheno
mena among which we are to look for illustrations or
explanations of the electromagnetic phenomena.
In describing the phenomena, I shall select those
which most clearly illustrate the fundamental ideas of
the theory, omitting others, or reserving them till the
reader is more advanced.
The most important aspect of any phenomenon from
a mathematical point of view is that of a measurable
quantity. I shall therefore consider electrical pheno
mena chiefly with a view to their measurement, de
scribing the methods of measurement, and defining
the standards on which they depend.
In the application of mathematics to the calculation
of electrical quantities, I shall endeavour in the first
place to deduce the most general conclusions from the
data at our disposal, and in the next place to apply
the results to the simplest cases that can be chosen.
I shall avoid, as much as I can, those questions which,
though they have elicited the skill of mathematicians,
have not enlarged our knowledge of science.
PREFACE. vii
The internal relations of the different branches of
the science which we have to study are more numerous
and complex than those of any other science hitherto
developed. Its external relations, on the one hand to
dynamics, and on the other to heat, light, chemical
action, and the constitution of bodies, seem to indicate
the special importance of electrical science as an aid
to the interpretation of nature.
It appears to me, therefore, that the study of elec-
tromagnetism in all its extent has now become of the
first importance as a means of promoting the progress
of science.
The mathematical laws of the different classes of
phenomena have been to a great extent satisfactorily
made out.
The connexions between the different classes of phe
nomena have also been investigated, and the proba
bility of the rigorous exactness of the experimental
laws has been greatly strengthened by a more extended
knowledge of their relations to each other.
Finally, some progress has been made in the re
duction of electromagnet-ism to a dynamical science,
by shewing that no electromagnetic phenomenon is
contradictory to the supposition that it depends on
purely dynamical action.
What has been hitherto done, however, has by no
means exhausted the field of electrical research. It
has rather opened up that field, by pointing out sub
jects of enquiry, and furnishing us with means of
investigation.
It is hardly necessary to enlarge upon the beneficial
viii PREFACE.
results of magnetic research on navigation, and the
importance of a knowledge of the true direction of
the compass, and of the effect of the iron in a ship.
But the labours of those who have endeavoured to
render navigation more secure by means of magnetic
observations have at the same time greatly advanced
the progress of pure science.
Gauss, as a member of the German Magnetic Union,
brought his powerful intellect to bear on the theory
of magnetism, and on the methods of observing it,
and he not only added greatly to our knowledge of
the theory of attractions, but reconstructed the whole
of magnetic science as regards the instruments used,
the methods of observation, and the calculation of the
results, so that his memoirs on Terrestrial Magnetism
may be taken as models of physical research by all
those who are engaged in the measurement of any
of the forces in nature.
The important applications of electromagnetism to
telegraphy have also reacted on pure science by giving
a commercial value to accurate electrical measure
ments, and by affording to electricians the use of
apparatus on a scale which greatly transcends that
of any ordinary laboratory. The consequences of this
demand for electrical knowledge, and of these experi
mental opportunities for acquiring it, have been already
very great, both in stimulating the energies of ad
vanced electricians, and in diffusing among practical
men a degree of accurate knowledge which is likely
to conduce to the general scientific progress of the
whole engineering profession.
PREFACE. ix
There are several treatises in which electrical and
magnetic phenomena are described in a popular way.
These, however, are not what is wanted by those wrho
have been brought face to face with quantities to be
measured, and whose minds do not rest satisfied with
lecture-room experiments.
There is also a considerable mass of mathematical
memoirs which are of great importance in electrical
science, but they lie concealed in the bulky Trans
actions of learned societies ; they do not form a con
nected system ; they are of very unequal merit, and
they are for the most part beyond the comprehension
of any but professed mathematicians.
I have therefore thought that a treatise would be
useful which should have for its principal object to
take up the whole subject in a methodical manner,
and which should also indicate how each part of the
subject is brought within the reach of methods of
verification by actual measurement.
The general complexion of the treatise differs con
siderably from that of several excellent electrical
works, published, most of them, in Germany, and it
may appear that scant justice is done to the specu
lations of several eminent electricians and mathema
ticians. One reason of this is that before I began
the study of electricity I resolved to read no mathe
matics on the subject till I had first read through
Faraday's Experimental Researches on Electricity. I
was aware that there was supposed to be a difference
between Faraday's way of conceiving phenomena and
that of the mathematicians, so that neither he nor
x PREFACE.
they were satisfied with each other's language. I had
also the conviction that this discrepancy did not arise
from either party being wrong. I was first convinced
of this by Sir William Thomson *, to whose advice and
assistance, as well as to his published papers, I owe
most of what I have learned on the subject.
As I proceeded with the study of Faraday, I per
ceived that his method of conceiving the phenomena
was also a mathematical one, though not exhibited
in the conventional form of mathematical symbols. I
also found that these methods were capable of being
expressed in the ordinary mathematical forms, and
thus compared with those of the professed mathema
ticians.
For instance, Faraday, in his mind's eye, saw lines
of force traversing all space where the mathematicians
saw centres of force attracting at a distance : Faraday
saw a medium where they saw nothing but distance :
Faraday sought the seat of the phenomena in real
actions going on in the medium, they were satisfied
that they had found it in a power of action at a
distance impressed on the electric fluids.
When I had translated what I considered to be
Faraday's ideas into a mathematical form, I found
that in general the results of the two methods coin
cided, so that the same phenomena were accounted
for, and the same laws of action deduced by both
methods, but that Faraday's methods resembled those
* I take this opportunity of acknowledging my obligations to Sir
W. Thomson and to Professor Tait for many valuable suggestions made
during the printing of this work.
PREFACE. xi
in which we begin with the whole and arrive at the
parts by analysis, while the ordinary mathematical
methods were founded on the principle of beginning
with the parts and building up the whole by syn
thesis.
I also found that several of the most fertile methods
of research discovered by the mathematicians could be
expressed much better in terms of ideas derived from
Faraday than in their original form.
The whole theory, for instance, of the potential, con
sidered as a quantity which satisfies a certain partial
differential equation, belongs essentially to the method
which I have called that of Faraday. According to
the other method, the potential, if it is to be considered
at all, must be regarded as the result of a summa
tion of the electrified particles divided each by its dis
tance from a given point. Hence many of the mathe
matical discoveries of Laplace, Poisson, Green and
Gauss find their proper place in this treatise, and their
appropriate expression in terms of conceptions mainly
derived from Faraday.
Great progress has been made in electrical science,
chiefly in Germany, by cultivators of the theory of
action at a distance. The valuable electrical measure
ments of W. Weber are interpreted by him according
to this theory, and the electromagnetic speculation
which was originated by Gauss, and carried on by
Weber, Riemann, J. and C. Neumann, Lorenz, &c. is
founded on the theory of action at a distance, but
depending either directly on the relative velocity of the
particles, or on the gradual propagation of something,
xii PREFACE.
whether potential or force, from the one particle to
the other. The great success which these eminent
men have attained in the application of mathematics
to electrical phenomena gives, as is natural, addi
tional weight to their theoretical speculations, so that
those who, as students of electricity, turn to them as
the greatest authorities in mathematical electricity,
would probably imbibe, along with their mathematical
methods, their physical hypotheses.
These physical hypotheses, however, are entirely
alien from the way of looking at things which I
adopt, and one object which I have in view is that
some of those who wish to study electricity may, by
reading this treatise, come to see that there is another
way of treating the subject, which is no less fitted to
explain the phenomena, and which, though in some
parts it may appear less definite, corresponds, as I
think, more faithfully with our actual knowledge, both
in what it affirms and in what it leaves undecided.
In a philosophical point of view, moreover, it is
exceedingly important that two methods should be
compared, both of which have succeeded in explaining
the principal electromagnetic phenomena, and both of
which have attempted to explain the propagation of
light as an electromagnetic phenomenon, and have
actually calculated its velocity, while at the same time
the fundamental conceptions of what actually takes
place, as well as most of the secondary conceptions of
the quantities concerned, are radically different.
I have therefore taken the part of an advocate rather
than that of a judge, and have rather exemplified one
PREFACE. xiii
method than attempted to give an impartial description
of both. I have no doubt that the method which I
have called the German one will also find its sup
porters, and will be expounded with a skill worthy
of its ingenuity.
I have not attempted an exhaustive account of elec
trical phenomena, experiments, and apparatus. The
student who desires to read all that is known on these
subjects will find great assistance from the Traite
d' Electricite of Professor A. de la Rive, and from several
German treatises, such as Wiedemann's Galvanismus,
Riess' Reibiingselektricitat, Beer's Einleitung in die Elek-
trostatik, &c.
I have confined myself almost entirely to the ma
thematical treatment of the subject, but I would
recommend the student, after he has learned, experi
mentally if possible, what are the phenomena to be
observed, to read carefully Faraday's Experimental
Researches in Electricity. He will there find a strictly
contemporary historical account of some of the greatest
electrical discoveries and investigations, carried on in
an order and succession which could hardly have been
improved if the results had been known from the
first, and expressed in the language of a man who
devoted much of his attention to the methods of ac
curately describing scientific operations and their re
sults *.
It is of great advantage to the student of any
subject to read the original memoirs on that subject,
for science is always most completely assimilated when
* Life and Letters of Faraday, vol. i. p. 395.
xiv PREFACE.
it is in the nascent state, and in the case of Faraday's
Researches this is comparatively easy, as they are
published in a separate form, and may be read con
secutively. If by anything I have here written I
may assist any student in understanding Faraday's
modes of thought and expression, I shall regard it as
the accomplishment of one of my principal aims — to
communicate to others the same delight which I have
found myself in reading Faraday's Researches.
The description of the phenomena, and the ele
mentary parts of the theory of each subject, will be
found in the earlier chapters of each of the four Parts
into which this treatise is divided. The student will
find in these chapters enough to give him an elementary
acquaintance with the whole science.
The remaining chapters of each Part are occupied
with the higher parts of the theory, the processes of
numerical calculation, and the instruments and methods
of experimental research.
The relations between electromagnetic phenomena
and those of radiation, the theory of molecular electric
currents, and the results of speculation on the nature
of action at a distance, are treated of in the last four
chapters of the second volume.
Feb. 1, 1873.
CONTENTS,
PRELIMINARY.
ON THE MEASUREMENT OF QUANTITIES.
Art. Page
1. The expression of a quantity consists of two factors, the nu
merical value, and the name of the concrete unit 1
2. Dimensions of derived units 1
3-5. The three fundamental units — Length, Time and Mass .. 2, 3
6. Derived units 5
7. Physical continuity and discontinuity 6
8. Discontinuity of a function of more than one variable .. .. 7
9. Periodic and multiple functions 8
10. Relation of physical quantities to directions in space .. .. 8
11. Meaning of the words Scalar and Vector 9
12. Division of physical vectors into two classes, Forces and Fluxes 10
13. Relation between corresponding vectors of the two classes .. 11
14. Line-integration appropriate to forces, surface-integration to
fluxes 12
15. Longitudinal and rotational vectors 12
16. Line-integrals and potentials 13
17. Hamilton's expression for the relation between a force and its
potential 15
18. Cyclic regions and geometry of position 16
19. The potential in an acyclic region is single valued 17
20. System of values of the potential in a cyclic region 18
21. Surface-integrals 19
22. Surfaces, tubes, and lines of flow 21
23. Right-handed and left-handed relations in space 24
24. Transformation of a line-integral into a surface-integral .. .. 25
25. Effect of Hamilton's operation v on a vector function .. .. 27
26. Xature of the operation v2 29
xvi CONTENTS.
PART I.
ELECTROSTATICS.
CHAPTER I.
DESCRIPTION OP PHENOMENA.
Art. Page
27. Electrification by friction. Electrification is of two kinds, to
which the names of Vitreous and Resinous, or Positive and
Negative, have been given .. .. 30
28. Electrification by induction 31
29. Electrification by conduction. Conductors and insulators .. 32
30. In electrification by friction the quantity of the positive elec
trification is equal to that of the negative electrification .. 33
31. To charge a vessel with a quantity of electricity equal and
opposite to that of an excited body 33
32. To discharge a conductor completely into a metallic vessel .. 34
33. Test of electrification by gold-leaf electroscope 34
34. Electrification, considered as a measurable quantity, may be
called Electricity 35
35. Electricity may be treated as a physical quantity 36
36. Theory of Two fluids .. 37
37. Theory of One fluid 39
38. Measurement of the force between electrified bodies 40
39. Relation between this force and the quantities of electricity .. 41
40. Variation of the force with the distance 42
41,42. Definition of the electrostatic unit of electricity. — Its
dimensions 42
43. Proof of the law of electric force 43
44. Electric field .. .... 44
45. Electric potential 45
46. Equipotential surfaces. Example of their use in reasoning
about electricity .. ,. .. 45
47. Lines of force 47
48. Electric tension 47
49. Electromotive force 47
50. Capacity of a conductor 48
51. Properties of bodies. —Resistance 48
CONTENTS. xvi i
Art. Pae*
52. Specific Inductive capacity of a dielectric 50
53. * Absorption' of electricity 50
54. Impossibility of an absolute charge ..51
55. Disruptive discharge. — Glow 52
56. Brush •• 54
57. Spark •• 55
58. Electrical phenomena of Tourmaline 56
59. Plan of the treatise, and sketch of its results 57
60. Electric polarization and displacement 59
61. The motion of electricity analogous to that of an incompressible
fluid 62
62. Peculiarities of the theory of this treatise 62
CHAPTER II.
ELEMENTAKY MATHEMATICAL THEORY OF ELECTRICITY.
63. Definition of electricity as a mathematical quantity .. .. .. 66
64. Volume-density, surface-density, and line-density .. .. .. 67
65. Definition of the electrostatic unit of electricity 68
66. Law of force between electrified bodies .. 69
67. Resultant force between two bodies 69
68. Resultant force at a point 69
69. Line-integral of electric force ; electromotive force 71
70. Electric potential 72
71. Resultant force in terms of the potential 72
72. The potential of all points of a conductor is the same .. .. 73
73. Potential due to an electrified system 74
74. Proof of the law of the inverse square 74
75. Surface-integral of electric induction 77
76. Introduction through a closed surface due to a single centre
of force 77
77. Poisson's extension of Laplace's equation .. ... 79
78. Conditions to be fulfilled at an electrified surface 80
79. Resultant force on an electrified surface 82
80. The electrification of a conductor is entirely on the surface .. 83
81. A distribution of electricity on lines or points is physically
impossible
82. Lines of electric induction 84
83. Specific inductive capacity 86
VOL. I. b
xviii CONTENTS.
CHAPTER III.
SYSTEMS OF CONDUCTORS.
Art.
84. On the superposition of electrified systems 88
85. Energy of an electrified system 88
86. General theory of a system of conductors. Coefficients of po
tential 89
87. Coefficients of induction. Capacity of a conductor. Dimensions
of these coefficients 90
88. Reciprocal property of the coefficients 91
89. A theorem due to Green 92
90. Relative magnitude of the coefficients of potential 92
91. And of induction 93
92. The resultant mechanical force on a conductor expressed in
terms of the charges of the different conductors of the system
and the variation of the coefficients of potential 94
93. The same in terms of the potentials, and the variation of the
coefficients of induction 94
94. Comparison of electrified systems 96
CHAPTER IV.
GENERAL THEOREMS.
95. Two opposite methods of treating electrical questions .. .. 98
96. Characteristics of the potential function 99
97. Conditions under which the volume-integral
dV dV a
vanishes 100
98. Thomson's theorem of the unique minimum of
1
^ («2-f o2 + <?2) dxdydz 103
99. Application of the theorem to the determination of the dis
tribution of electricity 107
100. Green's theorem and its physical interpretation 108
101. Green's functions 113
102. Method of finding limiting values of electrical coefficients .. 115
CONTENTS. XIX
CHAPTER V.
MECHANICAL ACTION BETWEEN ELECTRIFIED BODIES.
Art. Page
103. Comparison of the force between different electrified systems .. 119
104. Mechanical action on an element of an electrified surface .. 121
105. Comparison between theories of direct action and theories of
stress 122
106. The kind of stress required to account for the phenomeuou .. 123
107. The hypothesis of stress considered as a step in electrical
science 126
108. The hypothesis of stress shewn to account for the equilibrium
of the medium and for the forces acting between electrified
bodies 128
109. Statements of Faraday relative to the longitudinal tension and
lateral pressure of the lines of force 131
110. Objections to stress in a fluid considered 131
111. Statement of the theory of electric polarization 132
CHAPTER VI.
POINTS AND LINES OF EQUILIBRIUM,
112. Conditions of a point of equilibrium 135
113. Number of points of equilibrium 136
114. At a point or line of equilibrium there is a conical point or a
line of self-intersection of the equipotential surface .. .. 137
115. Angles at which an equipotential surface intersects itself .. 138
116. The equilibrium of an electrified body cannot be stable .. .. 139
CHAPTER VII.
FORMS OF EQUIPOTENTIAL SURFACES AND LINES OF FLOW.
117. Practical importance of a knowledge of these forms in simple
cases 142
118. Two electrified points, ratio 4 : 1. (Fig. I) 143
119. Two electrified points, ratio 4 : — 1. (Fig. II) 144
120. An electrified point in a uniform field offeree. (Fig. Ill) .. 145
121. Three electrified points. Two spherical equipotential sur
faces. (Fig. IV) 145
122. Faraday's use of the conception of lines of force 146
123. Method employed in drawing the diagrams 147
b 2
XX CONTENTS.
CHAPTER VIII.
SIMPLE CASES OF ELECTRIFICATION.
Art. 1'age
124. Two parallel planes 150
125. Two concentric spherical surfaces 152
126. Two coaxal cylindric surfaces 154
127. Longitudinal force on a cylinder, the ends of which are sur
rounded by cylinders at different potentials 155
CHAPTER IX.
SPHERICAL HARMONICS.
128. Singular points at which the potential becomes infinite .. .. 157
129. Singular points of different orders defined by their axes .. .. 158
130. Expression for the potential due to a singular point referred
to its axes .................... 160
131. This expression is perfectly definite and represents the most
general type of the harmonic of i degrees ........ 162
132. The zonal, tesseral, and sectorial types .......... 163
133. Solid harmonics of positive degree. Their relation to those
of negative degree .................. 165
134. Application to the theory of electrified spherical surfaces .. 166
135. The external action of an electrified spherical surface compared
with that of an imaginary singular point at its centre .. .. 167
136. Proof that if Yi and Y$ are two surface harmonics of different
degrees, the surface-integral / / Yi Yj dS = 0, the integration
being extended over the spherical surface ........ 169
137. Value of // Yi YjdS where YL and Yj are surface harmonics
of the same degree but of different types ........ 169
138. On conjugate harmonics ................ 170
139. If Yj is the zonal harmonic and Yi any other type of the
same degree
where Yi(j) is the value of Yi at the pole of Yj ...... 171
140. Development of a function in terms of spherical surface har
monics .................... ..172
141. Surface-integral of the square of a symmetrical harmonic .. 173
CONTENTS. xxi
Art. Page
142. Different methods of treating spherical harmonics 174
143. On the diagrams of spherical harmonics. (Figs. V, Vf, VII,
VHI, IX) .. 175
144. If the potential is constant throughout any finite portion of
space it is so throughout the whole region continuous with it
within which Laplace's equation is satisfied 176
145. To analyse a spherical harmonic into a system of conjugate
harmonics by means of a finite number of measurements at
selected points of the sphere 177
146. Application to spherical and nearly spherical conductors .. 178
CHAPTER X.
COXFOCAJL SURFACES OF THE SECOND DEGREE.
147. The lines of intersection of two systems and their intercepts
by the third system 181
148. The characteristic equation of V in terms of ellipsoidal co
ordinates 182
149. Expression of a, 0, y in terms of elliptic functions 183
150. Particular solutions of electrical distribution on the confocal
surfaces and their limiting forms 184
151. Continuous transformation into a figure of revolution about
the axis of « 187
152. Transformation into a figure of revolution about the axis of x 188
153. Transformation into a system of cones and spheres 189
154. Confocal paraboloids 189
CHAPTER XI.
THEORY OF ELECTRIC IMAGES.
155. Thomson's method of electric images 191
156. When two points are oppositely and unequally electrified, the
surface for which the potential is zero is a sphere .. .. 192
157. Electric images 193
158. Distribution of electricity on the surface of the sphere .. .. 195
1 59. Image of any given distribution of electricity 196
160. Resultant force between an electrified point and sphere .. .. 197
161. Images in an infinite plane conducting surface 198
162. Electric inversion 199
163. Geometrical theorems about inversion 201
164. Application of the method to the problem of Art. 158 .. .. 202
xxii CONTENTS.
Art. Page
165. Finite systems of successive images 203
166. Case of two spherical surfaces intersecting at an angle ^ ..204
167. Enumeration of the cases in which the number of images is
finite 206
168. Case of two spheres intersecting orthogonally 207
169. Case of three spheres intersecting orthogonally 210
170. Case of four spheres intersecting orthogonally 211
171. Infinite series of images. Case of two concentric spheres . . 212
172. Any two spheres not intersecting each other 213
173. Calculation of the coefficients of capacity and induction .. .. 216
174. Calculation of the charges of the spheres, and of the force
between them 217
175. Distribution of electricity on two spheres in contact. Proof
sphere 219
176. Thomson's investigation of an electrified spherical bowl .. .. 221
177. Distribution on an ellipsoid, and on a circular disk at po
tential V 221
178. Induction on an uninsulated disk or bowl by an electrified
point in the continuation of the plane or spherical surface .. 222
179. The rest of the sphere supposed uniformly electrified .. .. 223
180. The bowl maintained at potential V and uninfluenced .. .. 223
181. Induction on the bowl due to a point placed anywhere .. .. 224
CHAPTER XII.
CONJUGATE FUNCTIONS IN TWO DIMENSIONS.
182. Cases in which the quantities are functions of x and y only .. 226
183. Conjugate functions 227
184. Conjugate functions may be added or subtracted 228
185. Conjugate functions of conjugate functions are themselves
conjugate 229
186. Transformation of Poisson's equation 231
187. Additional theorems on conjugate functions 232
188. Inversion in two dimensions .. 232
189. Electric images in two dimensions 233
190. Neumann's transformation of this case 234
191. Distribution of electricity near the edge of a conductor formed
by two plane surfaces 236
192. Ellipses and hyperbolas. (Fig. X) 237
193. Transformation of this case. (Fig. XI) 238
CONTENTS. xxiu
Art.
194. Application to two cases of the flow of electricity in a con
ducting sheet .................... 239
195. Application to two cases of electrical induction ...... 239
196. Capacity of a condenser consisting of a circular disk between
two infinite planes .............. '-•*- •* - •• 240
197. Case of a series of equidistant planes cut off by a plane at right
angles to them .............. ,. •• •• 242
198. Case of a furrowed surface .............. 243
199. Case of a single straight groove ............ 243
200. Modification of the results when the groove is circular .. .. 244
201. Application to Sir W. Thomson's guard-ring ........ 245
202. Case of two parallel plates cut off by a perpendicular plane.
(Fig. XII) .................... 246
203. Case of a grating of parallel wires. • (Fig. XIII) ...... 248
204. Case of a single electrified wire transformed into that of the
grating ...................... 248
205. The grating used as a shield to protect a body from electrical
influence .................... 249
206. Method of approximation applied to the case of the grating .. 251
CHAPTER XIII.
ELECTROSTATIC INSTRUMENTS.
207. The frictional electrical machine 254
208. The electrophorus of Volta 255
209. Production of electrification by mechanical work. — Nicholson's
Revolving Doubler 256
210. Principle of Varley's and Thomson's electrical machines .. .. 256
211. Thomson's water-dropping machine ..259
212. Holtz's electrical machine 260
213. Theory of regenerators applied to electrical machines .. .. 260
214. On electrometers and electroscopes. Indicating instruments
and null methods. Difference between registration and mea
surement 262
215. Coulomb's Torsion Balance for measuring charges 263
216. Electrometers for measuring potentials. Snow Harris's and
Thomson's 266
217. Principle of the guard-ring. Thomson's Absolute Electrometer 267
218. Heterostatic method 269
219. Self-acting electrometers. — Thomson's Quadrant Electrometer 271
220. Measurement of the electric potential of a small body .. .. 274
221. Measurement of the potential at a point in the air 275
xxvi CONTENTS.
Art. Page
269. Dissipation of the ions and loss of polarization 321
270. Limit of polarization 321
271. Bitter's secondary pile compared with the Leyden jar .. .. 322
272. Constant voltaic elements. — Daniell's cell 325
CHAPTER VI.
MATHEMATICAL THEORY OF THE DISTRIBUTION OF ELECTRIC CURRENTS.
273. Linear conductors 329
274. Ohm's Law 329
275. Linear conductors in series 329
276. Linear conductors in multiple arc 330
277. Resistance of conductors of uniform section 331
278. Dimensions of the quantities involved in Ohm's law .. .. 332
279. Specific resistance and conductivity in electromagnetic measure 333
280. Linear systems of conductors in general 333
281. Reciprocal property of any two conductors of the system .. 335
282. Conjugate conductors .. .. 336
283. Heat generated in the system 336
284. The heat is a minimum when the current is distributed ac
cording to Ohm's law 337
CHAPTER VII.
CONDUCTION IN THREE DIMENSIONS.
285. Notation 338
286. Composition and resolution of electric currents 338
287. Determination of the quantity which flows through any surface 339
288. Equation of a surface of flow 340
289. Relation between any three systems of surfaces of flow .. .. 340
290. Tubes of flow 340
291. Expression for the components of the flow in terms of surfaces
offlow 341
292. Simplification of this expression by a proper choice of para
meters .. 341
293. Unit tubes of flow used as a complete method of determining
the current .. 341
294. Current-sheets and current-functions 342
295. Equation of ' continuity' 342
296. Quantity of electricity which flows through a given surface .. 344
CONTENTS. xxvii
CHAPTER VIII.
RESISTANCE AND CONDUCTIVITY IX THEEE DIMENSIONS.
Art. Page
297. Equations of resistance 345
298. Equations of conduction 346
299. Kate of generation of heat .. .. -.. ... *.. ".. .. 346
300. Conditions of stability - „ ... .. 347
301. Equation of continuity in a homogeneous medium 348
302. Solution of the equation 348
303. Theory of the coefficient T. It probably does not exist .. 349
304. Generalized form of Thomson's theorem .. .. 350
305. Proof without symbols 351
306. Strutt's method applied to a wire of variable section. — -Lower
limit of the value of the resistance 353
307. Higher limit 356
308. Lower limit for the correction for the ends of the wire .. .. 358
309. Higher limit 358
CHAPTER IX.
CONDUCTION THROUGH HETEROGENEOUS MEDIA.
310. Surface-conditions 360
311. Spherical surface 362
312. Spherical shell 363
313. Spherical shell placed in a field of uniform flow 364
314. Medium in which small spheres are uniformly disseminated .. 365
315. Images in a plane surface 366
316. Method of inversion not applicable in three dimensions .. .. 367
317. Case of conduction through a stratum bounded by parallel
planes 367
318. Infinite series of images. Application to magnetic induction .. 368
319. On stratified conductors. Coefficients of conductivity of a
conductor consisting of alternate strata of two different sub
stances 369
320. If neither of the substances has the rotatory property denoted
by I7 the compound conductor is free from it 370
321. If the substances are isotropic the direction of greatest resist
ance is normal to the strata 371
322. Medium containing parallelepipeds of another medium .. .. 371
323. The rotatory property cannot be introduced by means of con
ducting channels 372
324. Construction of an artificial solid having given coefficients of
longitudinal and transverse conductivity 373
xxviii CONTENTS.
CHAPTER X.
CONDUCTION IN DIELECTRICS.
Art. Page
325. In a strictly homogeneous medium there can be no internal
charge 374
326. Theory of a condenser in which the dielectric is not a perfect
insulator 375
327. No residual charge due to simple conduction 376
328. Theory of a composite accumulator 376
329. Residual charge and electrical absorption 378
330. Total discharge 380
331. Comparison with the conduction of heat 381
332. Theory of telegraph cables and comparison of the equations
with those of the conduction of heat 381
333. Opinion of Ohm on this subject 384
334. Mechanical illustration of the properties of a dielectric .. .. 385
CHAPTER XI.
MEASUREMENT OF THE ELECTRIC RESISTANCE OF CONDUCTORS.
335. Advantage of using material standards of resistance in electrical
measurements 388
336. Different standards which have been used and different systems
which have been proposed 388
337. The electromagnetic system of units .. 389
338. Weber's unit, and the British Association unit or Ohm .. ,. 389
339. Professed value of the Ohm 10,000,000 metres per second .. 389
340. Reproduction of standards 390
341. Forms of resistance coils 391
342. Coils of great resistance 392
343. Arrangement of coils in series 392
344. Arrangement in multiple arc 393
345. On the comparison of resistances. (1) Ohm's method .. .. 394
346. (2) By the differential galvanometer 394
347. (3) By Wheatstone's Bridge 398
348. Estimation of limits of error in the determination 399
349. Best arrangement of the conductors to be compared .. .. 400
350. On the use of Wheatstone's Bridge 402
351. Thomson's method for small resistances 404
352. Matthiessen and Hockin's method for small resistances ., •.. 406
CONTENTS. xxix
Art. Page
353. Comparison of great resistances by the electrometer .. .. 408
354. By accumulation in a condenser 409
355. Direct electrostatic method 409
356. Thomson's method for the resistance of a galvanometer .. .. 410
357. Mance's method of determining the resistance of a battery .. 411
358. Comparison of electromotive forces 413
CHAPTER XII.
ELECTRIC RESISTANCE OF SUBSTANCES.
359. Metals, electrolytes, and dielectrics 415
360. Resistance of metals 416
361. Resistance of mercury 417
362. Table of resistance of metals .. 418
363. Resistance of electrolytes 419
364. Experiments of Paalzow - 419
365. Experiments of Kohlrausch and Nippoldt ... 420
366. Resistance of dielectrics 421
367. Gutta-percha .. 423
368. Glass .. .. " .. ., 423
369. Gases .* 424
370. Experiments of Wiedemann and Riihlinann 425
ERRATA. VOL. I.
Page 26, 1. 3 from bottom, dele 'As we have made no assumption', &c.
down to 1. 7 of p. 27, 'the expression may then be written', and
substitute as follows : —
Let us now suppose that the curves for which a is constant
form a series of closed curves, surrounding the point on the surface
for which a has its minimum value, a0, the last curve of the series,
for which a = alt coinciding with the original closed curve s.
Let us also suppose that the curves for which /3 is constant form
a series of lines drawn from the point at which a = a0 to the
closed curve s, the first, /30, and the last, fa, being identical.
Integrating (8) by parts, the first term with respect to a and
the second with respect to /3, the double integrals destroy each
other. The line integral,
f^1 (X— '
/h dfi'
.
is zero, because the curve a = a0 is reduced to a point at which
there is but one value of X and of x.
The two line integrals,
*
destroy each other, because the point (a, /3J is identical with the
point (a, /30).
The expression (8) is therefore reduced to
Since the curve a = al is identical with the closed curve s, we
may write this expression
p. 80, in equations (3), (4), (6), (8), (17), (18), (19), (20), (21), (22), for
R read N.
p. 82, 1. 3, for Rl read Nl.
d-V d*V
p. 83, in equations (28), (29), (30), (31), for -^ read j^*
„ in equation (29), insert — before the second member.
p. 105, 1. 2, for Q read 8irQ.
p. 108, equation (1), for p read //.
„ „ (2), for p' read p.
„ „ (3), for a- read (/.
„ „ (4), for a-' read <r.
p. 113, 1. 4, for KR read ^- KR.
„ 1. 5, for KRRfcosc read --KRRfwse.
T: 7T
p. 114, 1. 5, for Sl read S.
p. 124, last line, for el-\-el read el + e2.
p. 125, lines 3 and 4, transpose within and without; 1. 16, for v
read V ; and 1. 18, for V read v.
p. 128, lines 11, 10, 8 from bottom, for dx read dz.
p. 149, 1. 24, for equpotential read equipotential.
•2 ERRATA. VOL. I.
p. 159, 1. 3, for F read f.
,, 1. 2 from bottom, for M read M2.
p. 163, 1. 20, for \i-s+i read AJ-^+I.
p. 164, equation (34), Jor (_iy-« J=— read (_!)*-*_
p. 179, equation (76), for i+l read 27+1.
X2 Z2 X2 Z2 '
p. 185, equation (24), for ~ — ~=l read -^ — T^r2 = 1-
p. 186, 1. 5 from bottom, for 'The surface-density on the elliptic plate'
read The surface-density on either side of the elliptic plate,
p. 186, equation (30), for 2n read 4ir.
p. 188, equation (38), for v2 read 2n2.
p. 196, 1. 27, for e..e read e1..e2.
Ee e2a3
p. 197, equation (10) should be M = — 1 —•
p. 204, 1. 15 from bottom, dele either,
p. 215, 1. 4, for \/2k read */2k.
E
p. 234, equation (13), for 2JZ read — •
p. 335, dele last 14 lines,
p. 336, 1. 1, dele therefore.
„ 1. 2, for l the potential at C to exceed that at D by P,' read a
current, (7, from X to Y.
„ 1. 4, for ' C to D will cause the potential at A to exceed that at
B by the same quantity P,' read X to Y will cause an equal
current G from A to B.
p. 351, 1. 3, for R2y? + R2v2 + R2w2 read R^
dt>
„ 1. 5, read +
p. 355, last line, for S' read S.
~db*
p. 356, equation (12), for — read ~
d
p. 365, in equations (12), (15), (16), for A read Ar.
E E
p. 366, equation (3), for ~ read — •
ri rz
p. 367, 1. 5, for 2^8 read 2k2S.
p. 368, equation (14), for <// read //.
p. 397, 1. 1, for ~8' read ~S'.
FJ rj
p. 404, at the end of Art. 350 insert as follows : —
When y, the resistance to be measured, a, the resistance of the
battery, and a, the resistance of the galvanometer, are given, the
best values of the other resistances have been shewn by Mr. Oliver
Heaviside (Phil. Mag., Feb. 1873) to be
a+y ' a+ y
ELECTRICITY AND MAGNETISM.
ELECTEICITY AND MAGNETISM.
PRELIMINARY.
ON THE MEASUREMENT OF QUANTITIES.
1.] EVERY expression of a Quantity consists of two factors or
components. One of these is the name of a certain known quan
tity of the same kind as the quantity to be expressed, which is
taken as a standard of reference. The other component is the
number of times the standard is to be taken in order to make up
the required quantity. The standard quantity is technically called
the Unit, and the number is called the Numerical Value of the
quantity.
There must be as many different units as there are different
kinds of quantities to be measured, but in all dynamical sciences
it is possible to define these units in terms of the three funda
mental units of Length, Time, and Mass. Thus the units of area
and of volume are defined respectively as the square and the cube
whose sides are the unit of length.
Sometimes, however, we find several units of the same kind
founded on independent considerations. Thus the gallon, or the
volume of ten pounds of water, is used as a unit of capacity as well
as the cubic foot. The gallon may be a convenient measure in
some cases, but it is not a systematic one, since its numerical re
lation to the cubic foot is not a round integral number.
2.] In framing a mathematical system we suppose the funda
mental units of length, time, and mass to be given, and deduce
all the derivative units from these by the simplest attainable de
finitions.
The formulae at which we arrive must be such that a person
VOL. i. B
2 PRELIMINARY. [3.
of any nation, by substituting for the different symbols the nu
merical value of the quantities as measured by his own national
units, would arrive at a true result.
Hence, in all scientific studies it is of the greatest importance
to employ units belonging to a properly defined system, and to
know the relations of these units to the fundamental units, so that
we may be able at once to transform our results from one system to
another.
This is most conveniently done by ascertaining the dimensions
of every unit in terms of the three fundamental units. When a
given unit varies as the ^th power of one of these units, it is said
to be of n dimensions as regards that unit.
For instance, the scientific unit of volume is always the cube
whose side is the unit of length. If the unit of length varies,
the unit of volume will vary as its third power, and the unit of
volume is said to be of three dimensions with respect to the unit of
length.
A knowledge of the dimensions of units furnishes a test which
ought to be applied to the equations resulting from any lengthened
investigation. The dimensions of every term of such an equa
tion, with respect to each of the three fundamental units, must
be the same. If not, the equation is absurd, and contains some
error, as its interpretation would be different according to the arbi
trary system of units which we adopt *.
The Three Fundamental Units.
3.] (1) Length. The standard of length for scientific purposes
in this country is one foot, which is the third part of the standard
yard preserved in the Exchequer Chambers.
In France, and other countries which have adopted the metric
system, it is the metre. The metre is theoretically the ten mil
lionth part of the length of a meridian of the earth measured
from the pole to the equator ; but practically it is the length of
a standard preserved in Paris, which was constructed by Borda
to correspond, when at the temperature of melting ice, with the
value of the preceding length as measured by Delambre. The metre
has not been altered to correspond with new and more accurate
measurements of the earth, but the arc of the meridian is estimated
in terms of the original metre.
* The theory of dimensions was first stated by Fourier, Tkeorie de Chaleur, § 160.
5-] THE THREE FUNDAMENTAL UNITS. 3
In astronomy the mean distance of the earth from the sun is
sometimes taken as a unit of length.
In the present state of science the most universal standard of
length which we could assume would be the wave length in vacuum
of a particular kind of light, emitted by some widely diffused sub
stance such as sodium, which has well-defined lines in its spectrum.
Such a standard would be independent of any changes in the di
mensions of the earth, and should be adopted by those who expect
their writings to be more permanent than that body.
In treating of the dimensions of units we shall call the unit of
length [£]. If I is the numerical value of a length, it is under
stood to be expressed in terms of the concrete unit [_Z/], so that
the actual length would be fully expressed by I \L~\.
4.] (2) Time. The standard unit of time in all civilized coun
tries is deduced from the time of rotation of the earth about its
axis. The sidereal day, or the true period of rotation of the earth,
can be ascertained with great exactness by the ordinary observa
tions of astronomers ; and the mean solar day can be deduced
from this by our knowledge of the length of the year.
The unit of time adopted in all physical researches is one second
of mean solar time.
In astronomy a year is sometimes used as a unit of time. A
more universal unit of time might be found by taking the periodic
time of vibration of the particular kind of light whose wave length
is the unit of length.
We shall call the concrete unit of time \_T~\, and the numerical
measure of time t.
5.] (3) Mass. The standard unit of mass is in this country the
avoirdupois pound preserved in the Exchequer Chambers. The
grain, which is often used as a unit, is defined to be the 7000th
part of this pound.
In the metrical system it is the gramme, which is theoretically
the mass of a cubic centimetre of distilled water at standard tem
perature and pressure, but practically it is the thousandth part
of a standard kilogramme preserved in Paris.
The accuracy with which the masses of bodies can be com
pared by weighing is far greater than that hitherto attained in
the measurement of lengths, so that all masses ought, if possible,
to be compared directly with the standard, and not deduced from
experiments on water.
In descriptive astronomy the mass of the sun or that of the
B 2
4 PRELIMINARY. [5.
earth is sometimes taken as a unit, but in the dynamical theory
of astronomy the unit of mass is deduced from the units of time
and length, combined with the fact of universal gravitation. The
astronomical unit of mass is that mass which attracts another
body placed at the unit of distance so as to produce in that body
the unit of acceleration.
In framing a universal system of units we may either deduce
the unit of mass in this way from those of length and time
already defined, and this we can do to a rough approximation in
the present state of science ; or, if we expect * soon to be able to
determine the mass of a single molecule of a standard substance,
we may wait for this determination before fixing a universal
standard of mass.
We shall denote the concrete unit of mass by the symbol \M ]
in treating of the dimensions of other units. The unit of mass
will be taken as one of the three fundamental units. When, as
in the French system, a particular substance,, water, is taken as
a standard of density, then the unit of mass is no longer inde
pendent, but varies as the unit of volume, or as [I/3].
If, as in the astronomical system, the unit of mass is defined
with respect to its attractive power, the dimensions of [If] are
[Z3?7-2].
For the acceleration due to the attraction of a mass m at a
fflL
distance r is by the Newtonian Law -j . Suppose this attraction
to act for a very small time t on a body originally at rest, and to
cause it to describe a space s, then by the formula of Galileo,
whence m = 2 — -^ . Since r and s are both lengths, and t is a
t
time, this equation cannot be true unless the dimensions of m are
[i/3^?7"2]. The same can be shewn from any astronomical equa
tion in which the mass of a body appears in some but not in all
of the terms f.
* See Prof. J. Loschmidt, ' Zur Grosse der Luftmolecule,' Academy of Vienna,)
Oct. 12, 1865; G. J. Stoney on 'The Internal Motions of Gases,' Phil. Mag., Aug.
1868 ; and Sir W. Thomson on • The Size of Atoms,' Nature, March 31, 1870.
'f If a foot and a second are taken as units, the astronomical unit of mass would
be about 932,000,000 pounds.
6.] DERIVED UNITS. 5
Derived Units.
6.] The unit of Velocity is that velocity in which unit of length
is described in unit of time. Its dimensions are [j&T7"1].
If we adopt the units of length and time derived from the
vibrations of light, then the unit of velocity is the velocity of
light.
The unit of Acceleration is that acceleration in which the velo
city increases by unity in unit of time. Its dimensions are [I/T~2].
The unit of Density is the density of a substance which contains
unit of mass in unit of volume. Its dimensions are [J/.Z/~3].
The unit of Momentum is the momentum of unit of mass moving
with unit of velocity. Its dimensions are [MLT~l~].
The unit of Force is the force which produces unit of momentum
in unit of time. Its dimensions are [MLT~2'].
This is the absolute unit of force, and this definition of it is
implied in every equation in Dynamics. Nevertheless, in many
text books in which these equations are given, a different unit of
force is adopted, namely, the weight of the national unit of mass ;
and then, in order to satisfy the equations, the national unit of mass
is itself abandoned, and an artificial unit is adopted as the dynamical
unit, equal to the national unit divided by the numerical value of
the force of gravity at the place. In this way both the unit of force
and the unit of mass are made to depend on the value of the
force of gravity, which varies from place to place, so that state
ments involving these quantities are not complete without a know
ledge of the force of gravity in the places where these statements
were found to be true.
The abolition, for all scientific purposes, of this method of mea
suring forces is mainly due to the introduction of a general system
of making observations of magnetic force in countries in which
the force of gravity is different. All such forces are now measured
according to the strictly dynamical method deduced from our
definitions, and the numerical results are the same in whatever
country the experiments are made.
The unit of Work is the work done by the unit of force acting
through the unit of length measured in its own direction. Its
dimensions are [J/Z2T~2].
The Energy of a system, being its capacity of performing work,
is measured by the work which the system is capable of performing
by the expenditure of its whole energy.
6 PRELIMINARY. [7.
The definitions of other quantities, and of the units to which
they are referred, will be given when we require them.
In transforming the values of physical quantities determined in
terms of one unit, so as to express them in terms of any other unit
of the same kind, we have only to remember that every expres
sion for the quantity consists of two factors, the unit and the nu
merical part which expresses how often the unit is to be taken.
Hence the numerical part of the expression varies inversely as the
magnitude of the unit, that is, inversely as the various powers of
the fundamental units which are indicated by the dimensions of the
derived unit.
On Physical Continuity and Discontinuity.
7.] A quantity is said to vary continuously when, if it passes
from one value to another, it assumes all the intermediate values.
We may obtain the conception of continuity from a consideration
of the continuous existence of a particle of matter in time and space.
Such a particle cannot pass from one position to another without
describing a continuous line in space, and the coordinates of its
position must be continuous functions of the time.
In the so-called ' equation of continuity,' as given in treatises
on Hydrodynamics, the fact expressed is that matter cannot appear
in or disappear from an element of volume without passing in or out
through the sides of that element.
A quantity is said to be a continuous function of its variables
when, if the variables alter continuously, the quantity itself alters
continuously.
Thus, if u is a function of x, and if, while x passes continuously
from #0 to fl?1} u passes continuously from n0 to ult but when x
passes from xl to #2, u passes from u-f to u2, %' being different from
%, then u is said to have a discontinuity in its variation with
respect to x for the value x = xl} because it passes abruptly from u^
to u{ while x passes continuously through #r
If we consider the differential coefficient of u with respect to x for
the value x = x^ as the limit of the fraction
when #2 and #0 are both made to approach ^ without limit, then,
if XQ and x2 are always on opposite sides of asly the ultimate value of
the numerator will be u^—uly and that of the denominator will
be zero. If u is a quantity physically continuous, the discontinuity
8.] CONTINUITY AND DISCONTINUITY. 7
can exist only with respect to the particular variable x. We must
in this case admit that it has an infinite differential coefficient
when x — XL. If u is not physically continuous, it cannot be dif
ferentiated at all.
It is possible in physical questions to get rid of the idea of
discontinuity without sensibly altering the conditions of the case.
If #0 is a very little less than xl} and x.2 a very little greater than
.r15 then UQ will be very nearly equal to u^ and u2 to u{. We
may now suppose u to vary in any arbitrary but continuous manner
from ?/0 to u2 between the limits XQ and x2. In many physical
questions we may begin with a hypothesis of this kind, and then
investigate the result when the values of #0 and x2 are made to
approach that of ^ and ultimately to reach it. The result will
in most cases be independent of the arbitrary manner in which we
have supposed u to vary between the limits.
Discontinuity of a Fimction of more than One Variable.
8.] If we suppose the values of all the variables except x to be
constant, the discontinuity of the function will occur for particular
values of #, and these will be connected with the values of the
other variables by an equation which we may write
$ = <£ (x, y, z3 &c.) = 0.
The discontinuity will occur when <f> = 0. When $ is positive the
function will have the form F2 (x} y, z, &c.). When <£ is negative
it will have the form F1 (x, y, z, &c.). There need be no necessary
relation between the forms F± and F2.
To express this discontinuity in a mathematical form, let one of
the variables, say .r, be expressed as a function of </> and the other
variables, and let F1 and F2 be expressed as functions of <£, y, z, &c.
We may now express the general form of the function by any
formula which is sensibly equal to F2 when <p is positive, 'and to
F± when c/> is negative. Such a formula is the following —
F— n<^ 2 •
As long as n is a finite quantity, however great, F will be a
continuous function, but if we make n infinite F will be equal to
F2 when <£ is positive, and equal to F^ when </> is negative.
Discontinuity of the Derivatives of a Continuous Function.
The first derivatives of a continuous function may be discon-
8 PRELIMINARY. [9.
tinuous. Let the values of the variables for which the discon
tinuity of the derivatives occurs be connected by the equation
<£ = <£(#,y, 2...) = 0,
and let FL and F2 be expressed in terms of $ and n—l other
variables, say (y> z . . .).
Then, when $ is negative, Fl is to be taken, and when $ is
positive F2 is to be taken, and, since F is itself continuous, when
</> is zero, F^ = F2.
Hence, when d> is zero, the derivatives — - and -~ may be
d(p dfy
different, but the derivatives with respect to any of the other
variables, such as —7^ and — =-£ must be the same. The discon-
du dy
t/ */
tinuity is therefore confined to the derivative with respect to 0, all
the other derivatives being- continuous.
Periodic and Multiple Functions.
9.] If u is a function of x such that its value is the same for
a?, x + at x + na, and all values of x differing by a, u is called a
periodic function of x, and a is called its period.
If x is considered as a function of u, then, for a given value of
U, there must be an infinite series of values of x differing by
multiples of a. In this case x is called a multiple function of u,
and a is called its cyclic constant.
dx
The differential coefficient -'— has only a finite number of values
du
corresponding to a given value of u.
On the Relation of Physical Quantities to Directions in Space.
10.] tln distinguishing the kinds of physical quantities, it is of
great importance to know how they are related to the directions
of those coordinate axes which we usually employ in defining the
positions of things. The introduction of coordinate axes into geo
metry by Des Cartes was one of the greatest steps in mathematical
progress, for it reduced the methods of geometry to calculations
performed on numerical quantities. The position of a point is made
to depend on the length of three lines which are always drawn in
determinate directions, and the line joining two points is in like
manner considered as the resultant of three lines.
But for many purposes in physical reasoning, as distinguished
II.] VECTORS, OR DIRECTED QUANTITIES. 9
from calculation, it is desirable to avoid explicitly introducing the
Cartesian coordinates, and to fix the mind at once on a point of
space instead of its three coordinates, and on the magnitude and
direction of a force instead of its three components. This mode
of contemplating geometrical and physical quantities is more prim
itive and more natural than the other, although the ideas connected
with it did not receive their full development till Hamilton made
the next great step in dealing with space, by the invention of his
Calculus of Quaternions.
As the methods of Des Cartes are still the most familiar to
students of science, and as they are really the most useful for
purposes of calculation, we shall express all our results in the
Cartesian form. I am convinced, however, that the introduction
of the ideas, as distinguished from the operations and methods of
Quaternions, will be of great use to us in the study of all parts
of our subject, and especially in electrodynamics, where we have to
deal with a number of physical quantities, the relations of which
to each other can be expressed far more simply by a few words of
Hamilton's, than by the ordinary equations.
11.] One of the most important features of Hamilton's method is
the division of quantities into Scalars and Vectors.
A Scalar quantity is capable of being completely defined by a
single numerical specification. Its numerical value does not in
any way depend on the directions we assume for the coordinate
axes.
A Vector, or Directed quantity, requires for its definition three
numerical specifications, and these may most simply be understood
as having reference to the directions of the coordinate axes.
Scalar quantities do not involve direction. The volume of a
geometrical figure, the mass and the energy of a material body,
the hydrostatical pressure at a point in a fluid, and the potential
at a point in space, are examples of scalar quantities.
A vector quantity has direction as well as magnitude, and is
such that a reversal of its direction reverses its sign. The dis
placement of a point, represented by a straight line drawn from
its original to its final position, may be taken as the typical
vector quantity, from which indeed the name of Vector is derived.
The velocity of a body, its momentum, the force acting on it,
an electric current, the magnetization of a particle of iron, are
instances of vector quantities.
There are physical quantities of another kind which are related
1 0 PRELIMINARY. [ I 2.
to directions in space, but which are not vectors. Stresses and
strains in solid bodies are examples of these, and the properties
of bodies considered in the theory of elasticity and in the theory
of double refraction. Quantities of this class require for their
definition nine numerical specifications. They are expressed in the
language of Quaternions by linear and vector functions of a vector.
The addition of one vector quantity to another of the same kind
is performed according to the rule given in Statics for the com
position of forces. In fact, the proof which Poisson gives of the
'parallelogram of forces' is applicable to the composition of any
quantities such that a reversal of their sign is equivalent to turning
them end for end.
When we wish to denote a vector quantity by a single symbol,
and to call attention to the fact that it is a vector, so that we must
consider its direction as well as its magnitude, we shall denote
it by a German capital letter, as fy, 33, &c.
In the calculus of Quaternions, the position of a point in space
is defined by the vector drawn from a fixed point, called the origin,
to that point. If at that point of space we have to consider any
physical quantity whose value depends on the position of the point,
that quantity is treated as a function of the vector drawn from
the origin. The function may be itself either scalar or vector.
The density of a body, its temperature, its hydrostatic pressure,
the potential at a point, are examples of scalar functions. The
resultant force at the point, the velocity of a fluid at that point,
the velocity of rotation of an element of the fluid, and the couple
producing rotation, are examples of vector functions.
12.] Physical vector quantities may be divided into two classes,
in one of which the quantity is defined with reference to a line,
while in the other the quantity is defined with reference to an
area.
For instance, the resultant of an attractive force in any direction
may be measured by finding the work which it would do on a
body if the body were moved a short distance in that direction
and dividing it by that short distance. Here the attractive force
is defined with reference to a line.
On the other hand, the flux of heat in any direction at any
point of a solid body may be defined as the quantity of heat which
crosses a small area drawn perpendicular to that direction divided
by that area and by the time. Here the flux is defined with
reference to an area.
13.] FORCES AND FLUXES. 11
There are certain cases in which a quantity may be measured
with reference to a line as well as with reference to an area.
Thus, in treating- of the displacements of elastic solids, we may
direct our attention either to the original and the actual position
of a particle, in which case the displacement of the particle is
measured by the line drawn from the first position to the second,
or we may consider a small area fixed in space, and determine
what quantity of the solid passes across that area during the dis
placement.
In the same way the velocity of a fluid may be investigated
either with respect to the actual velocity of the individual parti
cles, or with respect to the quantity of the fluid which flows through
any fixed area.
But in these cases we require to know separately the density of
the body as well as the displacement or velocity, in order to apply
the first method, and whenever we attempt to form a molecular
theory we have to use the second method.
In the case of the flow of electricity we do not know anything
of its density or its velocity in the conductor, we only know the
value of what, on the fluid theory, would correspond to the product
of the density and the velocity. Hence in all such cases we must
apply the more general method of measurement of the flux across
an area.
In electrical science, electromotive force and magnetic force
belong to the first class, being defined with reference to lines.
When we wish to indicate this fact, we may refer to them as
Forces.
On the other hand, electric and magnetic induction, and electric
currents, belong to the second class, being defined with reference
to areas. When we wish to indicate this fact, we shall refer to them
as Fluxes.
Each of these forces may be considered as producing, or tending
to produce, its corresponding flux. Thus, electromotive force pro
duces electric currents in conductors, and tends to produce them
in dielectrics. It produces electric induction in dielectrics, and pro
bably in conductors also. In the same sense, magnetic force pro
duces magnetic induction.
13.] In some cases the flux is simply proportional to the force
and in the same direction, but in other cases we can only affirm
that the direction and magnitude of the flux are functions of the
direction and magnitude of the force.
12 PRELIM1NAKY. [14.
The case in which the components of the flux are linear functions
of those of the force is discussed in the chapter on the Equations
of Conduction, Art. 296. There are in general nine coefficients
which determine the relation between the force and the flux. In
certain cases we have reason to believe that six of these coefficients
form three pairs of equal quantities. In such cases the relation be
tween the line of direction of the force and the normal plane of the
flux is of the same kind as that between a diameter of an ellipsoid
and its conjugate diametral plane. In Quaternion language, the
one vector is said to be a linear and vector function of the other, and
when there are three pairs of equal coefficients the function is said
to be self-conjugate.
In the case of magnetic induction in iron, the flux, (the mag
netization of the iron,) is not a linear function of the magnetizing
force. In all cases, however, the product of the force and the
flux resolved in its direction, gives a result of scientific import
ance, and this is always a scalar quantity.
14.] There are two mathematical operations of frequent occur
rence which are appropriate to these two classes of vectors, or
directed quantities.
In the case of forces, we have to take the integral along a line
of the product of an element of the line, and the resolved part of
the force along that element. The result of this operation is
called the Line-integral of the force. It represents the work
done on a body carried along the line. In certain cases in which
the line-integral does not depend on the form of the line, but
only on the position of its extremities, the line-integral is called
the Potential.
In the case of fluxes, we have to take the integral, over a surface,
of the flux through every element of the surface. The result of
this operation is called the Surface-integral of the flux. It repre
sents the quantity which passes through the surface.
There are certain surfaces across which there is no flux. If
two of these surfaces intersect, their line of intersection is a line
of flux. In those cases in which the flux is in the same direction
as the force, lines of this kind are often called Lines of Force. It
would be more correct, however, to speak of them in electrostatics
and magnetics as Lines of Induction, and in electrokinematics as
Lines of Flow.
15.] There is another distinction between different kinds of
directed quantities, which, though very important in a physical
1 6.] LINE-INTEGRALS. 1 3
point of view, is not so necessary to be observed for the sake of
the mathematical methods. This is the distinction between longi
tudinal and rotational properties.
The direction and magnitude of a quantity may depend upon
some action or effect which takes place entirely along a certain
line, or it may depend upon something of the nature of rota
tion about that line as an axis. The laws of combination of
directed quantities are the same whether they are longitudinal or
rotational, so that there is no difference in the mathematical treat
ment of the two classes, but there may be physical circumstances
which indicate to which class we must refer a particular pheno
menon. Thus, electrolysis consists of the transfer of certain sub
stances along a line in one direction, and of certain other sub
stances in the opposite direction, which is evidently a longitudinal
phenomenon, and there is no evidence of any rotational effect
about the direction of the force. Hence we infer that the electric
current which causes or accompanies electrolysis is a longitudinal,
and not a rotational phenomenon.
On the other hand, the north and south poles of a magnet do
not differ as oxygen and hydrogen do, which appear at opposite
places during electrolysis, so that we have no evidence that mag
netism is a longitudinal phenomenon, while the effect of magnetism
in rotating the plane of polarized light distinctly shews that mag
netism is a rotational phenomenon.
On Line-integrals.
16.] The operation of integration of the resolved part of a vector
quantity along a line is important in physical science generally,
and should be clearly understood.
Let X) y, z be the coordinates of a point P on a line whose
length, measured from a certain point A, is s. These coordinates
will be functions of a single variable s.
Let R be the value of the vector quantity at P, and let the
tangent to the curve at P make with the direction of R the angle e,
then .72 cose is the resolved part of R along the line, and the
integral
C*
= /
^o
R cos e
is called the line-integral of R along the line s.
We may write this expression
14 PRELIMINARY. [l6.
where X, T, Z are the components of R parallel to #, y, z respect
ively.
This quantity is, in general, different for different lines drawn
between A and P. When, however, within a certain region, the
quantity
Xdx+ Ydy + Zdz = -DV,
that is, is an exact differential within that region, the value of L
becomes Jj = \^A—\^P)
and is the same for any two forms of the path between A and P,
provided the one form can be changed into the other by con
tinuous motion without passing out of this region.
On Potentials.
The quantity ^ is a scalar function of the position of the point,
and is therefore independent of the directions of reference. It is
called the Potential Function, and the vector quantity whose com
ponents are X, Y, Z is said to have a potential ^, if
*--©• '--($• '--<£>• •
When a potential function exists, surfaces for which the po
tential is constant are called Equipotential surfaces. The direction
of R at any point of such a surface coincides with the normal to
dty
the surface, and if n be a normal at the point P. then R = =- •
dn
The method of considering the components of a vector as the
first derivatives of a certain function of the coordinates with re
spect to these coordinates was invented by Laplace * in his treat
ment of the theory of attractions. The name of Potential was first
given to this function by Green f, who made it the basis of his
treatment of electricity. Green's essay was neglected by mathe
maticians till 1846, and before that time most of its important
theorems had been rediscovered by Gauss, Chasles, Sturm, and
Thomson J.
In the theory of gravitation the potential is taken with the
opposite sign to that which is here used, and the resultant force
in any direction is then measured by the rate of increase of the
* Mec. Celeste, liv. iii.
t Essay on the Application of Mathematical Analysis to the Theories of Electricity
and Magnetism, Nottingham, 1828. Eeprinted in Crelle's Journal, and in Mr. Ferrer's
edition of Green's Works.
J Thomson and Tait, Natural Philosophy, § 483.
17.] RELATION BETWEEN FORCE AND POTENTIAL. 15
potential function in that direction. In electrical and magnetic
investigations the potential is defined so that the resultant force
in any direction is measured by the decrease of the potential in
that direction. This method of using the expression makes it
correspond in sign with potential energy, which always decreases
when the bodies are moved in the direction of the forces acting
on them.
17.] The geometrical nature of the relation between the poten
tial and the vector thus derived from it receives great light from
Hamilton's discovery of the form of the operator by which the vector
is derived from the potential.
The resolved part of the vector in any direction is, as we have
seen, the first derivative of the potential with respect to a co
ordinate drawn in that direction, the sign being reversed.
Now if it /, k are three unit vectors at right angles to each
other, and if X, Y, Z are the components of the vector 5 resolved
parallel to these vectors, then
9 = IX+jY+kZ; (1)
and by what we have said above, if ^ is the potential,
If we now write V for the operator,
(4)
The symbol of operation V may be interpreted as directing us
to measure, in each of three rectangular directions, the rate of
increase of '&, and then, considering the quantities thus found as
vectors, to compound them into one. This is what we are directed
to do by the expression (3). But we may also consider it as directing
us first to find out in what direction ^ increases fastest, and then
to lay off in that direction a vector representing this rate of
increase.
M. Lame, in his Traite des Fonctions Inverses, uses the term
Differential Parameter to express the magnitude of this greatest
rate of increase, but neither the term itself, nor the mode in which
Lame uses it, indicates that the quantity referred to has direction
as well as magnitude. On those rare occasions in which I shall have
to refer to this relation as a purely geometrical one, I shall call the
vector g the Slope of the scalar function ^, using the word Slope
16 PRELIMINARY. [l8.
to indicate the direction, as well as the magnitude, of the most
rapid decrease of #.
18.] There are cases, however, in which the conditions
dZ dY dX dZ dY dX
— ;- = 0, -j — = 0, and — =- = 0,
dy dz dz ax dx dy
which are those of Xdx + Ydy -f Zdz being a complete differential,
are fulfilled throughout a certain region of space, and yet the line-
integral from A to P may be different for two lines, each of
which lies wholly within that region. This may be the case if
the region is in the form of a ring, and if the two lines from A
to P pass through opposite segments of the ring. In this case,
the one path cannot be transformed into the other by continuous
motion without passing out of the region.
We are here led to considerations belonging to the Geometry
of Position, a subject which, though its importance was pointed
out by Leibnitz and illustrated by Gauss, has been little studied.
The most complete treatment of this subject has been given by
J. B. Listing *.
Let there be p points in space, and let I lines of any form be
drawn joining these points so that no two lines intersect each
other, and no point is left isolated. We shall call a figure com
posed of lines in this way a Diagram. Of these lines, p — 1 are
sufficient to join the p points so as to form a connected system.
Every new line completes a loop or closed path, or, as we shall
call it, a Cycle. The number of independent cycles in the diagram
is therefore K = I — jo+1.
Any closed path drawn along the lines of the diagram is com
posed of these independent cycles, each being taken any number of
times and in either direction.
The existence of cycles is called Cyclosis, and the number of
cycles in a diagram is called its Cyclomatic number.
Cyclosis in Surfaces and Regions.
Surfaces are either complete or bounded. Complete surfaces are
either infinite or closed. Bounded surfaces are limited by one or
more closed lines, which may in the limiting cases become finite
lines or points.
A finite region of space is bounded by one or more closed
surfaces. Of these one is the external surface, the others are
* Der Census RaumlicTier Complexe, Gott. Abh., Bd. x. S. 97 (1861).
19.] CYCLIC REGIONS. 17
included in it and exclude each other, and are called internal
surfaces.
If the region has one bounding- surface, we may suppose that
surface to contract inwards without breaking its continuity or
cutting itself. If the region is one of simple continuity, such as
a sphere, this process may be continued till it is reduced to a
point; but if the region is like a ring, the result will be a closed
curve; and if the region has multiple connexions, the result will
be a diagram of lines, and the cyclomatic number of the diagram
will be that of the region. The space outside the region has the
same cyclomatic number as the region itself. Hence, if the region
is bounded by internal as well as external surfaces, its cyclomatic
number is the sum of those due to all the surfaces.
When a region encloses within itself other regions, it is called a
Periphractic region.
The number of internal bounding surfaces of a region is called
its periphractic number. A closed surface is also periphractic, its
number being unity.
The cyclomatic number of a closed surface is twice that of the
region which it bounds. To find the cyclomatic number of a
bounded surface, suppose all the boundaries to contract inwards,
without breaking continuity, till they meet. The surface will then
be reduced to a point in the case of an acyclic surface, or to a linear
diagram in the case of cyclic surfaces. The cyclomatic number of
the diagram is that of the surface.
19.] THEOREM I. If throughout any acyclic region
Xdx+Ydy + Zch =-£*,
the value of the line-integral from a point A to a point P taken
along any path within the region will le the same.
We shall first shew that the line-integral taken round any closed
path within the region is zero.
Suppose the equipotential surfaces drawn. They are all either
closed surfaces or are bounded entirely by the surface of the region,
so that a closed line within the region, if it cuts any of the sur
faces at one part of its path, must cut the same surface in the
opposite direction at some other part of its path, and the corre
sponding portions of the line-integral being equal and opposite,
the total value is zero.
Hence if AQP and AQ'P are two paths from A to P, the line-
integral for AQ'P is the sum of that for AQP and the closed path
VOL. i. c
18 PRELIMINARY. [20.
AQ'PQA. But the line-integral of the closed path is zero, there
fore those of the two paths are equal.
Hence if the potential is given at any one point of such a
region, that at any other point is determinate.
20.] THEOREM II. In a cyclic region in which the equation
Xdx+Ydy + Zdz = -D*
is everywhere fulfilled, the line-integral from, A to P, along a
line drawn within the region, will not in general be determinate
unless the channel of communication between A and P be specified,
Let K be the cyclomatic number of the region, then K sections
of the region may be made by surfaces which we may call Dia
phragms, so as to close up K of the channels of communication,
and reduce the region to an acyclic condition without destroying
its continuity.
The line-integral from A to any point P taken along a line
which does not cut any of these diaphragms will be, by the last
theorem, determinate in value.
Now let A and P be taken indefinitely near to each other, but
on opposite sides of a diaphragm, and let K be the line-integral
from A to P.
Let A and P' be two other points on opposite sides of the same
diaphragm and indefinitely near to each other, and let K' be the
line-integral from A' to F. Then K'= K.
For if we draw AA' and PP7, nearly coincident, but on opposite
sides of the diaphragm, the line-integrals aloug these lines will be
equal. Suppose each equal to Z, then the line-integral of A'P* is
equal to that of A'A + AP + PP= -L+K+L = K = that of AP.
Hence the line-integral round a closed curve which passes through
one diaphragm of the system in a given direction is a constant
quantity K. This quantity is called the Cyclic constant corre
sponding to the given cycle.
Let any closed curve be drawn within the region, and let it cut
the diaphragm of the first cycle p times in the positive direction
and p' times in the negative direction, and let p —p = % . Then
the line-integral of the closed curve will be % Kr
Similarly the line-integral of any closed curve will be
where nK represents the excess of the number of positive passages
of the curve through the diaphragm of the cycle K over the
number of negative passages.
21.] SURFACE-INTEGRALS. 19
If two curves are such that one of them may be transformed
into the other by continuous motion without at any time passing*
through any part of space for which the condition of having a
potential is not fulfilled,, these two curves are called Reconcileabje
curves. Curves for which this transformation cannot be effected
are called Irreconcileable curves *.
The condition that Xdx+Ydy -\- Zdz is a complete differential
of some function ^ for all points within a certain region, occurs in
several physical investigations in which the directed quantity and
the potential have different physical interpretations.
In pure kinematics we may suppose X, Y, Z to be the com
ponents of the displacement of a point of a continuous body whose
original coordinates are x, y, z, then the condition expresses that
these displacements constitute a non-rotational strain f.
If X, Y, Z represent the components of the velocity of a fluid at
the point x, y, z, then the condition expresses that the motion of the
fluid is irrotational.
If X, Y, Z represent the components of the force at the point
#, i/j z, then the condition expresses that the work done on a
particle passing from one point to another is the difference of the
potentials at these points, and the value of this difference is the
same for all reconcileable paths between the two points.
On Surface-Integrals.
21.] Let dS be the element of a surface, and c the angle which
a normal to the surface drawn towards the positive side of the
surface makes with the direction of the vector quantity R, then
R cos tdS is called the surface-integral of It over the surface S.
ff
THEOREM III. The surface-integral of the flux through a closed
surface may be expressed as the volume-integral of its convergence
taken within the surface. (See Art. 25.)
Let X, Y, Z be the components of R, and let I, m, n be the
direction-cosines of the normal to S measured outwards. Then the
surface-integral of R over S is
{(R cos e dS = ffxidS + JJYmdS + j j ZndS
= IJXdydz+jJYdzdx -f- ft Zdxdy, (1)
* See Sir W. Thomson C0n Vortex Motion,' Trans. R. S. Edin., 1869.
t See Thomson and Tait's Natural Philosophy, § 190 (»).
C 2
20 PRELIMINARY. [21.
the values of X, Y, Z being those at a point in the surface, and
the integrations being extended over the whole surface.
If the surface is a closed one, then, when y and z are given,
the coordinate sc must have an even number of values, since a line
parallel to x must enter and leave the enclosed space an equal
number of times provided it meets the surface at all.
Let a point travelling from # = — oo to # = +oo first enter
the space when os = asl9 then leave it when x = a?2, and so on;
and let the values of X at these points be X1 , X2 , &c., then
,- X3) + &c. 4 (li.-.Xi.-,)} <fy«fe. (2)
If Jf is a quantity which is continuous, and has no infinite values
between and #2, then
dX
where the integration is extended from the first to the second
intersection, that is, along the first segment of x which is within
the closed surface. Taking into account all the segments which lie
within the closed surface, we find
the double integration being confined to the closed surface, but
the triple integration being extended to the whole enclosed space.
Hence, if X, J, Z are continuous and finite within a closed surface
$, the total surface-integral of R over that surface will be
IT*, ffr/dX dY dz\ -
JJ*™***-JJJfc + ^ + jg)*** (5)
the triple integration being extended over the whole space within &
Let us next suppose that X, Y, Z are not continuous within the
closed surface, but that at a certain surface F (x, y> z] = 0 the
values of X} Yy Z alter abruptly from X, Y, Z on the negative side
of the surface to X', Y' ', Z' on the positive side.
If this discontinuity occurs, say, between a?t and #2, the value
J x. dx
where in the expression under the integral sign only the finite
values of the derivative of X are to be considered.
In this case therefore the total surface-integral of R over the
closed surface will be expressed by
22.] SOLENOIDAL DISTRIBUTION. 21
+
jj(Y'-Y)dzdx + ff(Z'-Z)dxdy; (7)
or, if /', m', ft' are the direction-cosines of the normal to the surface
of discontinuity, and dS' an element of that surface,
', (8)
where the integration of the last term is to be extended over the
surface of discontinuity.
If at every point where X, Y, Z are continuous
dX dY dZ
^ + ^ + ^ = °> (9)
and at every surface where they are discontinuous
I'X' + m' T + n'Z' = I' X+ m' Y+ n'Z, (10)
then the surface-integral over every closed surface is zero, and the
distribution of the vector quantity is said to be Solenoidal.
We shall refer to equation (9) as the General solenoidal con
dition, and to equation (10) as the Superficial solenoidal condition.
22.] Let us now consider the case in which at every point
within the surface 8 the equation
«+£ + *?.0 (11)
dx dy dz
is fulfilled. We have as a consequence of this the surface-integral
over the closed surface equal to zero.
Now let the closed surface S consist of three parts Slt S0, and
S2. Let S1 be a surface of any form bounded by a closed line Lr
Let SQ be formed by drawing lines from every point of L± always
coinciding with the direction of E. If I, m, n are the direction-
cosines of the normal at any point of the surface $0, we have
RcoB* = Xl+Ym + Zn = 0. (12)
Hence this part of the surface contributes nothing towards the
value of the surface-integral.
Let #2 be another surface of any form bounded by the closed
curve L.2 in which it meets the surface S0.
Let Q1} Q0, Q2 be the surface-integrals of the surfaces SISS0,S2,
and let Q be the surface-integral of the closed surface S. Then
22 PRELIMINARY. [2,2.
and we know that QQ = 0 ; (14)
therefore Q2 = - Ql ; (15)
or, in other words, the surface-integral over the surface $2 is equal
and opposite to that over SL whatever be the form and position
of £2, provided that the intermediate surface S0 is one for which R
is always tangential.
If we suppose Z^ a closed curve of small area,, $0 will be a
tubular surface having the property that the surface-integral over
every complete section of the tube is the same.
Since the whole space can be divided into tubes of this kind
provided dX dY dZ
-T- + -T- + -j- = 0, (16)
dx du dz '
J
a distribution of a vector quantity consistent with this equation is
called a Solenoidal Distribution.
On Tubes and Lines of Flow.
If the space is so divided into tubes that the surface-integral
for every tube is unity, the tubes are called Unit tubes, and the
surface-integral over any finite surface S bounded by a closed
curve L is equal to the number of such tubes which pass through
S in the positive direction, or, what is the same thing, the number
which pass through the closed curve L.
Hence the surface-integral of S depends only on the form of
its boundary L, and not on the form of the surface within its
boundary.
On Peripkractic Regions.
If, throughout the whole region bounded externally by the single
closed surface Slt the solenoidal condition
dX dY dZ^_
dx dv dz
t/
is fulfilled, then the surface-integral taken over any closed surface
drawn within this region will be zero, and the surface-integral
taken over a bounded surface within the region will depend only
on the form of the closed curve which forms its boundary.
It is not, however, generally true that the same results follow
if the region within which the solenoidal condition is fulfilled is
bounded otherwise than by a single surface.
For if it is bounded by more than one continuous surface, one of
these is the external surface and the others are internal surfaces,
22.] PERIPHRACTIC REGIONS. 23
and the region S is a periphractic region, having within it other
regions which it completely encloses.
If within any of these enclosed regions, S^ the solenoidal con
dition is not fulfilled, let
*-//
R cos e dSl
be the surface-integral for the surface enclosing this region, and
let Q2, Q3, &c. be the corresponding quantities for the other en
closed regions.
Then, if a closed surface S' is drawn within the region St the
value of its surface-integral will be zero only when this surface
/S" does not include any of the enclosed regions S19 S2, &c. If it
includes any of these, the surface-integral is the sum of the surface-
integrals of the different enclosed regions which lie within it.
For the same reason, the surface-integral taken over a surface
bounded by a closed curve is the same for such surfaces only bounded
by the closed curve as are reconcileable with the given surface by
continuous motion of the surface within the region S.
When we have to deal with a periphractic region, the first thing
to be done is to reduce it to an aperiphractic region by drawing
lines joining the different bounding surfaces. Each of these lines,
provided it joins surfaces which were not already in continuous
connexion, reduces the periphractic number by unity, so that the
whole number of lines to be drawn to remove the periphraxy is
equal to the periphractic number, or the number of internal sur
faces. When these lines have been drawn we may assert that if
the solenoidal condition is fulfilled in the region S, any closed surface
drawn entirely within S, and not cutting any of the lines, has its
surface-integral zero.
In drawing these lines we must remember that any line joining
surfaces which are already connected does not diminish the peri
phraxy, but introduces cyclosis.
The most familiar example of a periphractic region within which
the solenoidal condition is fulfilled is the region surrounding a mass
attracting or repelling inversely as the square of the distance.
In this case we have
/>* y* g
X = m — > Y= m — > Z = m — ;
r3 r3 r3
where m is the mass supposed to be at the origin of coordinates.
At any point where r is finite
dX dY dZ
T + -j- + -r = °»
ax ay dz
24 PRELIMINARY. [23.
but at the origin these quantities become infinite. For any closed
surface not including the origin, the surface-integral is zero. If
a closed surface includes the origin, its surface-integral is 4?m.
If, for any reason, we wish to treat the region round m as if it
were not periphractic, we must draw a line from m to an infinite
distance, and in taking surface-integrals we must remember to add
4Trm whenever this line crosses from the negative to the positive
side of the surface.
On Right-handed and Left-handed Relations in Space.
23.] In this treatise the motions of translation along any axis
and of rotation about that axis, will be assumed to be of the same
sign when their directions correspond to those of the translation
and rotation of an ordinary or right-handed screw *.
For instance, if the actual rotation of the earth from west to east
is taken positive, the direction of the earth's axis from south to
north will be taken positive, and if a man walks forward in the
positive direction, the positive rotation is in the order, head, right-
hand, feet, left-hand.
If we place ourselves on the positive side of a surface, the positive
direction along its bounding curve will be opposite to the motion
of the hands of a watch with its face towards us.
This is the right-handed system which is adopted in Thomson
and Tait's Natural Philosophy, § 243. The opposite, or left-handed
system, is adopted in Hamilton's and Tait's Quaternions. The
operation of passing from the one system to the other is called, by
Listing, Perversion.
The reflexion of an object in a mirror is a perverted image of the
object.
"When we use the Cartesian axes of %, y, z, we shall draw them
* The combined action of the muscles of the arm when we turn the upper side of
the right-hand outwards, and at the same time thrust the hand forwards, will
impress the right-handed screw motion on the memory more firmly than any verbal
definition. A common corkscrew may be used as a material symbol of the same
relation.
Professor W. H. Miller has suggested to me that as the tendrils of the vine are
right-handed screws and those of the hop left-handed, the two systems of relations in
space might be called those of the vine and the hop respectively.
The system of the vine, which we adopt, is that of Linnseus, and of screw-makers
in all civilized countries except Japan. De Candolle was the first who called the
hop-tendril right-handed, and in this he is followed by Listing, and by most writers
on the rotatory polarization of light. Screws like the hop-tendril are made for the
couplings of railway-carriages, and for the fittings of wheels on the left side of ordinary
carriages, but they are always called left-handed screws by those who use them.
24.] LINE-INTEGRAL AND SURFACE-INTEGRAL. 25
so that the ordinary conventions about the cyclic order of the
symbols lead to a right-handed system of directions in space. Thus,
if x is drawn eastward and y northward, z must be drawn upward.
The areas of surfaces will be taken positive when the order of
integration coincides with the cyclic order of the symbols. Thus,
the area of a closed curve in the plane of xy may be written either
\xdy
or —
the order of integration being x, y in the first expression, and y^ x
in the second.
This relation between the two products dx dy and dy d-x may
be compared with that between the products of two perpendicular
vectors in the doctrine of Quaternions, the sign of which depends
on the order of multiplication, and with the reversal of the sign
of a determinant when the adjoining rows or columns are ex
changed.
For similar reasons a volume-integral is to be taken positive when
the order of integration is in the cyclic order of the variables xt y, z,
and negative when the cyclic order is reversed.
We now proceed to prove a theorem which is useful as esta
blishing a connexion between the surface-integral taken over a
finite surface and a line-integral taken round its boundary.
24.] THEOREM IV. A line-integral taken round a closed curve
may be expressed in terms of a surface-integral taken over a
surface bounded by the curve.
Let X, Y} Z be the components of a vector quantity 51 whose line-
integral is to be taken round a closed curve s.
Let S be any continuous finite surface bounded entirely by the
closed curve s, and let f, 77, f be the components of another vector
quantity 33, related to X, Y, Z by the equations
_
dy dz ~ dz dx ~ dx dy
Then the surface-integral of §3 taken over the surface S is equal to
the line-integral of 51 taken round the curve s. It is manifest that
£, YJ, f fulfil of themselves the so'lenoidal condition
d( drj dC
_z i __ I _j __ ± __ o.
dx dy dz
Let /, m, n be the direction-cosines of the normal to an element
26 PEELIMINARY. [24.
of the surface dSt reckoned in the positive direction. Then the
value of the surface-integral of 33 may be written
(2)
In order to form a definite idea of the meaning of the element
dS, we shall suppose that the values of the coordinates x, y, z for
every point of the surface are given as functions of two inde
pendent variables a and p. If ft is constant and a varies, the point
(>, y, z) will describe a curve on the surface, and if a series of values
is given to /3, a series of such curves will be traced, all lying on
the surface S. In the same way, by giving a series of constant
values to a, a second series of curves may be traced, cutting the
first series, and dividing the whole surface into elementary portions,
any one of which may be taken as the element dS.
The projection of this element on the plane of y, z is, by the
ordinary formula,
, 70 ,dy dz dy dz^ _ . *
IdS = (-T- TS - -A -T-) dP da- (3)
\»a dp dp da'
The expressions for mdS and ndS are obtained from this by sub
stituting x, y> z in cyclic order.
The surface-integral which we have to find is
(4)
or, substituting the values of £, 77, f in terms of X, Y, Z,
dX dX dY dY dZ dZ
The part of this which depends on X may be written
dXsdzdx dz dx dX ,dx dy dx dy
~
, ,. , , . dXdx dx . . .
adding and subtracting -= — 7 - — , this becomes
3 dx da dp
f C ( dx ,dX dx dX dy dX dz^
JJ (dp \da da dy da dz do)
dx ,dX dx dX dy dX ,
~^" + ~ + ~ P a'
As we have made no assumption as to the form of the functions
a and /3, we may assume that a is a function of X, or, in other
words, that the curves for which a is constant are those for which
25.] HAMILTON'S OPERATOR v. 27
7 1?"
X is constant. In this case -^- = 0, and the expression becomes
dp
by integration with respect to a,
C CdX dx -, f ^r dx ,
/ / dQda. = X-T-d8; (9) •
JJ da dp ' J dp
where the integration is now to be performed round the closed
curve. Since all the quantities are now expressed in terms of one
variable & we may make s, the length of the bounding curve, the
independent variable, and the expression may then be written
AS* <io>
where the integration is to be performed round the curve s. We
may treat in the same way the parts of the surface -integral which
depend upon T and Z, so that we get finally,
where the first integral is extended over the surface £, and the
second round the bounding curve s *.
On the effect of the operator V on a vector function.
• 25.] We have seen that the operation denoted by V is that by
which a vector quantity is deduced from its potential. The same
operation, however, when applied to a vector function, produces
results which enter into the two theorems we have just proved
(III and IV). The extension of this operator to vector displacements,
and most of its further development, is due to Professor Tait f.
Let o- be a vector function of p, the vector of a variable point.
Let us suppose, as usual, that
p = ix+jy + kz,
and o- = iX+jY+kZ;
where Xy Y, Z are the components of o- in the directions of the
axes.
We have to perform on cr the operation
. d . d 7 d
V = i-j- +J-T +b-r'
dx ay dz
Performing this operation, and remembering the rules for the
* This theorem was given by Professor Stokes. Smith's Prize Examination, 1854,
question 8. It is proved in Thomson and Tait's Natural Philosophy, § 190 (f).
t See Proc. R. S. Edin., April 28, 1862. • On Green's and other allied Theorems,'
Trans. R. S. Edin., 1869-70, a very valuable paper ; and ' On some Quaternion
Integrals,' Proc. R. S. Edin., 1870-71.
28 PRELIMINARY. [25.
multiplication of i, /, /£, we find that V a- consists of two parts,
one scalar and the other vector.
The scalar part is
,dX dY dZ. TTT
#V <T = — (-7- + -j- + -j-}t see Theorem III,
\dx dy dz'
and the vector part is
.(dZ dY. ,,dX d2\ /./^_^\
* VJJT ~ ~3i) + <? \dz~~fa>+ ^ ~dgJ
If the relation between X, Y} Z and £, r/, f is that given by
equation (1) of the last theorem, we may write
F V o- = & £+ y rj + Jc C See Theorem IV.
It appears therefore that the functions of X, Y, Z which occur
in the two theorems are both obtained by the operation V on the
vector whose components are X, Y, Z. The theorems themselves
may be written
jjjSVvds =jfs.vUvds, (III)
and fsvdp =JJ8.V<FUvd9i (IV)
where d 9 is an element of a volume, ds of a surface, dp of a curve,
and Uv a unit- vector in the direction of the normal.
To understand the meaning1 of these functions of a vector, let us
suppose that o-0 is the value of o- at a point P, and let us examine
the value of o- — o-0 in the neighbourhood of P.
If we draw a closed surface round P} then, if the
I/ surface-integral of o- over this surface is directed
inwards, S V o- will be positive, and the vector
p O-—O-Q near the point P will be on the whole
/ » X^ directed towards P, as in the figure (1).
I propose therefore to call the scalar part of
jv i V (T the convergence of o- at the point P.
To interpret the vector part of Vo-, let us
suppose ourselves to be looking in the direction of the vector
whose components are f, 77, £ and let us examine
-* — the vector o- — o-0 near the point P. It will appear
I p. as in the figure (2), this vector being arranged on
the whole tangentially in the direction opposite to
the hands of a watch.
I propose (with great diffidence) to call the vector
part of V o- the curl, or the version of o- at the point P.
26.] CONCENTRATION. 29
At Fig. 3 we have an illustration of curl combined with con
vergence.
Let us now consider the meaning of the equation ,
VV a = 0. \
This implies that V <r is a scalar, or that the vector X
o- is the slope of some scalar function $?. These f
applications of the operator V are due to Professor ^ „
Tait *. A more complete development of the theory
is given in his paper ' On Green's and other allied Theorems f/
to which I refer the reader for the purely Quaternion investigation
of the properties of the operator V.
26.] One of the most remarkable properties of the operator V is
that when repeated it becomes
an operator occurring in all parts of Physics, which we may refer to
as Laplace's Operator.
This operator is itself essentially scalar. When it acts on a
scalar function the result is scalar, when it acts on a vector function
the result is a vector.
If, with any point P as centre, we draw a small sphere whose
radius is r, then if q0 is the value of q at the centre, and q the
mean value of q for all points within the sphere,
2o-2 = iV>-2v22;
so that the value at the centre exceeds or falls short of the mean
value according as V2 q is positive or negative.
I propose therefore to call V2<? the concentration of q at the
point P, because it indicates the excess of the value of q at that
point over its mean value in the neighbourhood of the point.
If q is a scalar function, the method of finding its mean value is
well known. If it is a vector function, we must find its mean
value by the rules for integrating vector functions. The result
of course is a vector.
* Proceedings R. S. Edin., 1862. t Trans. R. 8. Edin., 1869-70.
PART I.
ELECTROSTATICS.
CHAPTEE I.
DESCRIPTION OF PHENOMENA.
Electrification by Friction.
27.] EXPERIMENT I *". Let a piece of glass and a piece of resin,
neither of which exhibits any electrical properties, be rubbed to
gether and left with the rubbed surfaces in contact. They will
still exhibit no electrical properties. Let them be separated. They
will now attract each other.
If a second piece of glass be rubbed with a second piece of
resin, and if the pieces be then separated and suspended in the
neighbourhood of the former pieces of glass and resin,, it may be
observed —
(1) That the two pieces of glass repel each other.
(2) That each piece of glass attracts each piece of resin.
(3) That the two pieces of resin repel each other.
These phenomena of attraction and repulsion are called Elec
trical phenomena, and the bodies which exhibit them are said to
be electrified, or to be charged with electricity.
Bodies may be electrified in many other ways, as well as by
friction.
The electrical properties of the two pieces of glass are similar
to each other but opposite to those of the two pieces of resin,
the glass attracts what the resin repels and repels what the resin
attracts.
* See Sir W. Thomson ' On the Mathematical Theory of Electricity,' Cambridge
and Dublin Mathematical Journal, March, 1848.
28.] ELECTRIFICATION. 31
If a body electrified in any manner whatever behaves as the
glass does, that is, if it repels the glass and attracts the resin, the
body is said to be vitreously electrified, and if it attracts the glass
and repels the resin it is said to be resinously electrified. All
electrified bodies are found to be either vitreously or resinously
electrified.
It is the established practice of men of science to call the vitreous
electrification positive, and the resinous electrification negative.
The exactly opposite properties of the two kinds of electrification
justify us in indicating them by opposite signs, but the applica
tion of the positive sign to one rather than to the other kind must
be considered as a matter of arbitrary convention, just as it is a
matter of convention in mathematical diagrams to reckon positive
distances towards the right hand.
No force, either of attraction or of repulsion, can be observed
between an electrified body and a body not electrified. When, in
any case, bodies not previously electrified are observed to be acted
on by an electrified body, it is because they have become electrified
by induction.
Electrification by Induction.
28.] EXPERIMENT II *. Let a hollow vessel of metal be hung
up by white silk threads, and let a similar thread
be attached to the lid of the vessel so that the vessel
may be opened or closed without touching it.
Let the pieces of glass and resin be similarly sus
pended and electrified as before.
Let the vessel be originally unelectrified, then if
an electrified piece of glass is hung up within it by
its thread without touching the vessel, and the lid
closed, the outside of the vessel will be found to
be vitreously electrified, and it may be shewn that
the electrification outside of the vessel is exactly the
same in whatever part of the interior space the glass
is suspended.
If the glass is now taken out of the vessel without touching it,
the electrification of the glass will be the same as before it was
put in, and that of the vessel will have disappeared.
This electrification of the vessel, which depends on the glass
* This, and several experiments which follow, are due to Faraday, ' On Static
Electrical Inductive Action,1 Phil. Mag., 1843, or Exp. Res., vol. ii. p. 279.
32 ELECTROSTATIC PHENOMENA. [29.
being within it, and which vanishes when the glass is removed, is
called Electrification by induction.
Similar effects would be produced if the glass were suspended
near the vessel on the outside, but in that case we should find
an electrification vitreous in one part of the outside of the vessel
and resinous in another. When the glass is inside the vessel
the whole of the outside is vitreously and the whole of the inside
resinously electrified.
Electrification ly Conduction.
29.] EXPERIMENT III. Let the metal vessel be electrified by
induction, as in the last experiment, let a second metallic body
be suspended by white silk threads near it, and let a metal wire,
similarly suspended, be brought so as to touch simultaneously the
electrified vessel and the second body.
The second body will now be found to be vitreously electrified,
and the vitreous electrification of the vessel will have diminished.
The electrical condition has been transferred from the vessel to
the second body by means of the wire. The wire is called a con
ductor of electricity, and the second body is said to be electrified
by conduction.
Conductors and Insulators.
EXPERIMENT IV. If a glass rod, a stick of resin or gutta-percha,
or a white silk thread, had been used instead of the metal wire, no
transfer of electricity would have taken place. Hence these latter
substances are called Non-conductors of electricity. Non-conduc
tors are used in electrical experiments to support electrified bodies
without carrying off their electricity. They are then called In
sulators.
The metals are good conductors ; air, glass, resins, gutta-percha,
vulcanite, paraffin, &c. are good insulators; but, as we shall see
afterwards, all substances resist the passage of electricity, and all
substances allow it to pass, though in exceedingly different degrees.
This subject will be considered when we come to treat of the
Motion of electricity. For the present we shall consider only two
classes of bodies, good conductors, and good insulators.
In Experiment II an electrified body produced electrification in
the metal vessel while separated from it by air, a non-conducting
medium. Such a medium, considered as transmitting these electrical
effects without conduction, has been called by Faraday a Dielectric
31.] SUMMATION OF ELECTRIC EFFECTS. 33
medium, and the action which takes place through it is called
Induction.
In Experiment III the electrified vessel produced electrification
in the second metallic body through the medium of the wire. Let
us suppose the wire removed, and the electrified piece of glass taken
out of the vessel without touching it, and removed to a sufficient
distance. The second body will still exhibit vitreous electrifica
tion, but the vessel, when the glass is removed, will have resinous
electrification. If we now bring the wire into contact with both
bodies, conduction will take place along the wire, and all electri
fication will disappear from both bodies, shewing that the elec
trification of the two bodies was equal and opposite.
30.] EXPERIMENT V. In Experiment II it was shewn that if
a piece of glass, electrified by rubbing it with resin, is hung up in
an insulated metal vessel, the electrification observed outside does
not depend on the position of the glass. If we now introduce the
piece of resin with which the glass was rubbed into the same vessel,
without touching it or the vessel, it will be found that there is
no electrification outside the vessel. From this we conclude that
the electrification of the resin is exactly equal and opposite to that
of the glass. By putting in any number of bodies, electrified in
any way, it may be shewn that the electrification of the outside of
the vessel is that due to the algebraic sum of all the electrifica
tions, those being reckoned negative which are resinous. We have
thus a practical method of adding the electrical effects of several
bodies without altering the electrification of each.
31.] EXPERIMENT VI. Let a second insulated metallic vessel, J5,
be provided, and let the electrified piece of glass be put into the
first vessel A, and the electrified piece of resin into the second vessel
B. Let the two vessels be then put in communication by the metal
wire, as in Experiment III. All signs of electrification will dis
appear.
Next, let the wire be removed, and let the pieces of glass and of
resin be taken out of the vessels without touching them. It will
be found that A is electrified resinously and B vitreously.
If now the glass and the vessel A be introduced together into a
larger insulated vessel C, it will be found that there is no elec
trification outside C. This shews that the electrification of A is
exactly equal and opposite to that of the piece of glass, and that
of B may be shewn in the same way to be equal and opposite to that
of the piece of resin.
VOL. I. D
34 ELECTROSTATIC PHENOMENA. [32.
We have thus obtained a method of charging a vessel with a
quantity of electricity exactly equal and opposite to that of an
electrified body without altering the electrification of the latter,
and we may in this way charge any number of vessels with exactly
equal quantities of electricity of either kind, which we may take
for provisional units.
32.] EXPERIMENT VII. Let the vessel B, charged with a quan
tity of positive electricity, which we shall call, for the present,
unity, be introduced into the larger insulated vessel C without
touching it. It will produce a positive electrification on the out
side of C. Now let B be made to touch the inside of C. No change
of the external electrification will be observed. If B is now taken
out of C without touching it, and removed to a sufficient distance,
it will be found that B is completely discharged, and that C has
become charged with a unit of positive electricity.
We have thus a method of transferring the charge of B to C.
Let B be now recharged with a unit of electricity, introduced
into C already charged, made to touch the inside of C, and re
moved. It will be found that B is again completely discharged,
so that the charge of C is doubled.
If this process is repeated, it will be found that however highly
C is previously charged, and in whatever way B is charged, when
B is first entirely enclosed in C, then made to touch C, and finally
removed without touching C, the charge of B is completely trans
ferred to C, and B is entirely free from electrification.
This experiment indicates a method of charging a body with
any number of units of electricity. We shall find, when we come
to the mathematical theory of electricity, that the result of this
experiment affords an accurate test of the truth of the theory.
33.] Before we proceed to the investigation of the law of
electrical force, let us enumerate the facts we have already esta
blished.
By placing any electrified system inside an insulated hollow con
ducting vessel, and examining the resultant effect on the outside
of the vessel, we ascertain the character of the total electrification
of the system placed inside, without any communication of elec
tricity between the different bodies of the system.
The electrification of the outside of the vessel may be tested
with great delicacy by putting it in communication with an elec
troscope.
We may suppose the electroscope to consist of a strip of gold
34-] ELECTRICITY AS A QUANTITY. 35
leaf hanging between two bodies charged, one positively, and the
other negatively. If the gold leaf becomes electrified it will incline
towards the body whose electrification is opposite to its own. By
increasing the electrification of the two bodies and the delicacy of
the suspension, an exceedingly small electrification of the gold leaf
may be detected.
When we come to describe electrometers and multipliers we
shall find that there are still more delicate methods of detecting
electrification and of testing the accuracy of our theorems, but at
present we shall suppose the testing to be made by connecting the
hollow vessel with a gold leaf electroscope.
This method was used by Faraday in his very admirable de
monstration of the laws of electrical phenomena *.
34.] I. The total electrification of a body, or system of bodies,
remains always the same, except in so far as it receives electrifi
cation from or gives electrification to other bodies.
In all electrical experiments the electrification of bodies is found
to change, but it is always found that this change is due to want
of perfect insulation, and that as the means of insulation are im
proved, the loss of electrification becomes less. We may therefore
assert that the electrification of a body placed in a perfectly in
sulating medium would remain perfectly constant.
II. When one body electrifies another by conduction, the total
electrification of the two bodies remains the same, that is, the one
loses as much positive or gains as much negative electrification as
the other gains of positive or loses of negative electrification.
For if the two bodies are enclosed in the hollow vessel, no change
of the total electrification is observed.
III. When electrification is produced by friction, or by any
other known method, equal quantities of positive and negative elec
trification are produced.
For the electrification of the whole system may be tested in
the hollow vessel, or the process of electrification may be carried
on within the vessel itself, and however intense the electrification of
the parts of the system may be, the electrification of the whole,
as indicated by the gold leaf electroscope, is invariably zero.
The electrification of a body is therefore a physical quantity
capable of measurement, and two or more electrifications can be
combined experimentally with a result of the same kind as when
* ' On Static Electrical Inductive Action,' Phil. Mag., 1843, or Exp. Res., vol. ii.
p. 249.
D 2
36 ELECTROSTATIC PHENOMENA. [35.
two quantities are added algebraically. We therefore are entitled
to use language fitted to deal with electrification as a quantity as
well as a quality, and to speak of any electrified body as ( charged
with a certain quantity of positive or negative electricity/
35.] While admitting electricity, as we have now done, to the
rank of a physical quantity, we must not too hastily assume that
it is, or is not, a substance, or that it is, or is not, a form of
energy, or that it belongs to any known category of physical
quantities. All that we have hitherto proved is that it cannot
be created or annihilated, so that if the total quantity of elec
tricity within a closed surface is increased or diminished, the in
crease or diminution must have passed in or out through the closed
surface.
This is true of matter, and is expressed by the equation known as
the Equation of Continuity in Hydrodynamics.
It is not true of heat, for heat may be increased or diminished
within a closed surface, without passing in or out through the
surface, by the transformation of some other form of energy into
heat, or of heat into some other form of energy.
It is not true even of energy in general if we admit the imme
diate action of bodies at a distance. For a body outside the closed
surface may make an exchange of energy with a body within
the surface. But if all apparent action at a distance is the
result of the action between the parts of an intervening medium,
and if the nature of this action of the parts of the medium is
clearly understood, then it is conceivable that in all cases of the
increase or diminution of the energy within a closed surface we
may be able to trace the passage of the energy in or out through
that surface.
There is, however, another reason which warrants us in asserting
that electricity, as a physical quantity, synonymous with the total
electrification of a body, is not, like heat, a form of energy. An
electrified system has a certain amount of energy, and this energy
can be calculated by multiplying the quantity of electricity in
each of its parts by another physical quantity, called the Potential
of that part, and taking half the sum of the products. The quan
tities ' Electricity ' and ' Potential,' when multiplied together,
produce the quantity 'Energy.' It is impossible, therefore, that
electricity and energy should be quantities of the same category, for
electricity is only one of the factors of energy, the other factor
being « Potential.'
36.] THEORIES OF ELECTRICITY. 37
Energy, which is the product of these factors, may also be con
sidered as the product of several other pairs of factors, such as
A Force x A distance through which the force is to act.
A Mass X Gravitation acting through a certain height.
A Mass x Half the square of its velocity.
A Pressure X A volume of fluid introduced into a vessel at
that pressure.
A Chemical Affinity x A chemical change, measured by the number
of electro-chemical equivalents which enter
into combination.
If we obtain distinct mechanical ideas of the nature of electric
potential, we may combine these with the idea of energy to
determine the physical category in which ' Electricity ' is to be
placed.
36.] In most theories on the subject, Electricity is treated as
a substance, but inasmuch as there are two kinds of electrification
which, being combined, annul each other, and since we cannot
conceive of two substances annulling each other, a distinction has
been drawn between Free Electricity and Combined Electricity.
Theory of Two Fluids.
In the theory called that of Two Fluids, all bodies, in their
unelectrified state, are supposed to be charged with equal quan
tities of positive and negative electricity. These quantities are
supposed to be so great that no process of electrification has ever
yet deprived a body of all the electricity of either kind. The pro
cess of electrification, according to this theory, consists in taking
a certain quantity P of positive electricity from the body A and
communicating it to B, or in taking a quantity N of negative
electricity from B and communicating it to A^ or in some com
bination of these processes.
The result will be that A will have P + N units of negative
electricity over and above its remaining positive electricity, which
is supposed to be in a state of combination with an equal quantity
of negative electricity. This quantity P + N is called the Free
electricity, the rest is called the Combined, Latent, or Fixed elec
tricity.
In most expositions of this theory the two electricities are called
' Fluids,' because they are capable of being transferred from one
body to another, and are, within conducting bodies, extremely
38 ELECTROSTATIC PHENOMENA. [36.
mobile. The other properties of fluids, such as their inertia,
weight, and elasticity, are not attributed to them by those who
have used the theory for merely mathematical purposes ; but the
use of the word Fluid has been apt to mislead the vulgar, including1
many men of science who are not natural philosophers, and who
have seized on the word Fluid as the only term in the statement
of the theory which seemed intelligible to them.
We shall see that the mathematical treatment of the subject has
been greatly developed by writers who express themselves in terms
of the ' Two Fluids ' theory. . Their results, however, have been
deduced entirely from data which can be proved by experiment,
and which must therefore be true, whether we adopt the theory of
two fluids or not. The experimental verification of the mathe
matical results therefore is no evidence for or against the peculiar
doctrines of this theory.
The introduction of two fluids permits us to consider the negative
electrification of A and the positive electrification of B as the effect
of any one of three different processes which would lead to the same
result. We have already supposed it produced by the transfer of
P units of positive electricity from A to B, together with the
transfer of N units of negative electricity from B to A. But if
P+N units of positive electricity had been transferred from A
to B, or if P + N units of negative electricity had been transferred
from B to A, the resulting ' free electricity ' on A and on B would
have been the same as before, but the quantity of ' combined
electricity' in A would have been less in the second case and greater
in the third than it was in the first.
It would appear therefore, according to this theory, that it is
possible to alter not only the amount of free electricity in a body,
but the amount of combined electricity. But no phenomena have
ever been observed in electrified bodies which can be traced to the
varying amount of their combined electricities. Hence either the
combined electricities have no observable properties, or the amount
of the combined electricities is incapable of variation. The first
-of these alternatives presents no difficulty to the mere mathema
tician, who attributes no properties to the fluids except those of
.attraction and repulsion, for in this point of view the two fluids
simply annul one another, and their combination is a true mathe
matical zero. But to those who cannot use the word Fluid without
thinking of a substance it is difficult to conceive that the com
bination of the two fluids shall have no properties at all, so that
37-] THEORIES OF ONE AND OP TWO FLUIDS. 39
the addition of more or less of the combination to a body shall not
in any way affect it, either by increasing- its mass or its weight, or
altering some of its other properties. Hence it has been supposed
by some, that in every process of electrification exactly equal quan
tities of the two fluids are transferred in opposite directions, so
that the total quantity of the two fluids in any body taken to
gether remains always the same. By this new law they ' contrive
to save appearances,' forgetting that there would have been no need
of the law except to reconcile the ' two fluids ' theory with facts,
and to prevent it from predicting non-existent phenomena.
Theory of One Fluid.
37.] In the theory of One Fluid everything is the same as in
the theory of Two Fluids except that, instead of supposing the two
substances equal and opposite in all respects, one of them, gene
rally the negative one, has been endowed with the properties and
name of Ordinary Matter, while the other retains the name of The
Electric Fluid. The particles of the fluid are supposed to repel
one another according to the law of the inverse square of the
distance, and to attract those of matter according to the same
law. Those of matter are supposed to repel each other and attract
those of electricity. The attraction, however, between units of the
different substances at unit of distance is supposed to be a very little
greater than the repulsion between units of the same kind, so that
a unit of matter combined with a unit of electricity will exert a
force of attraction on a similar combination at a distance, this
force, however, being exceedingly small compared with the force
between two uncombined units.
This residual force is supposed to account for the attraction of
gravitation. Unelectrified bodies are supposed to be charged with
as many units of electricity as they contain of ordinary matter.
When they contain more electricity or less, they are said to be
positively or negatively electrified.
This theory does not, like the Two-Fluid theory, explain too
much. It requires us, however, to suppose the mass of the electric
fluid so small that no attainable positive or negative electrification
has yet perceptibly increased or diminished either the mass or the
weight of a body, and it has not yet been able to assign sufficient
reasons why the vitreous rather than the resinous electrification
should be supposed due to an excess of electricity.
One objection has sometimes been urged against this theory by
40 ELECTROSTATIC PHENOMENA. [38.
men who ought to have reasoned better. It has been said that
the doctrine that the particles of matter uncombined with elec
tricity repel one another, is in direct antagonism with the well-
established fact that every particle of matter attracts every other
particle throughout the universe. If the theory of One Fluid were
true we should have the heavenly bodies repelling one another.
But it is manifest that the heavenly bodies, according to this
theory, if they consisted of matter uncombined with electricity,
would be in the highest state of negative electrification, and would
repel each other. We have no reason to believe that they are in
such a highly electrified state, or could be maintained in that
state. The earth and all the bodies whose attraction has been
observed are rather in an unelectrified state, that is, they contain
the normal charge of electricity, and the only action between them
is the residual force lately mentioned. The artificial manner, how
ever, in which this residual force is introduced is a much more
valid objection to the theory.
In the present treatise I propose, at different stages of the in
vestigation, to test the different theories in the light of additional
classes of phenomena. For my own part, I look for additional
light on the nature of electricity from a study of what takes place
in the space intervening between the electrified bodies. Such is the
essential character of the mode of investigation pursued by Faraday
in his Experimental Researches, and as we go on I intend to exhibit
the results, as developed by Faraday, W. Thomson, &c., in a con
nected and mathematical form, so that we may perceive what
phenomena are explained equally well by all the theories, and what
phenomena indicate the peculiar difficulties of each theory.
Measurement of the Force between Electrified Bodies.
38.] Forces may be measured in various ways. For instance,
one of the bodies may be suspended from one arm of a delicate
balance, and weights suspended from the other arm, till the body,
when unelectrified, is in equilibrium. The other body may then
be placed at a known distance beneath the first, so that the
attraction or repulsion of the bodies when electrified may increase
or diminish the apparent weight of the first. The weight which
must be added to or taken from the other arm, when expressed
in dynamical measure, will measure the force between the bodies.
This arrangement was used by Sir W. Snow Harris, and is that
adopted in Sir W. Thomson's absolute electrometers. See Art. 217.
39-] MEASUREMENT OF ELECTRIC FORCES. 41
It is sometimes more convenient to use a torsion-balance in
which a horizontal arm is suspended by a fine wire or fibre, so as
to be capable of vibrating about the vertical wire as an axis, and
the body is attached to one end of the arm and acted on by the
force in the tangential direction, so as to turn the arm round the
vertical axis, and so twist the suspension wire through a certain
angle. The torsional rigidity of the wire is found by observing
the time of oscillation of the arm, the moment of inertia of the
arm being otherwise known, and from the angle of torsion and
the torsional rigidity the force of attraction or repulsion can be
deduced. The torsion-balance was devised by Michell for the de
termination of the force of gravitation between small bodies, and
was used by Cavendish for this purpose. Coulomb, working in
dependently of these philosophers, reinvented it, and successfully
applied it to discover the laws of electric and magnetic forces;
and the torsion-balance has ever since been used in all researches
where small forces have to be measured. See Art. 215.
39.] Let us suppose that by either of these methods we can
measure the force between two electrified bodies. We shall suppose
the dimensions of the bodies small compared with the distance
between them, so that the result may not be much altered by
any inequality of distribution of the electrification on either body,
and we shall suppose that both bodies are so suspended in air as
to be at a considerable distance from other bodies on which they
might induce electrification.
It is then found that if the bodies are placed at a fixed distance
and charged respectively with e and e' of our provisional units of
electricity, they will repel each other with a force proportional
to the product of e and /. If either e or e' is negative, that is,
if one of the charges is vitreous and the other resinous, the force
will be attractive, but if both e and e' are negative the force is again
repulsive.
We may suppose the first body, A, charged with m units of
vitreous and n units of resinous electricity, which may be con
ceived separately placed within the body, as in Experiment V.
Let the second body, B, be charged with m' units of positive
and n' units of negative electricity.
Then each of the m positive units in A will repel each of the m
positive units in B with a certain force, say f, making a total effect
equal to mm'f.
Since the effect of negative electricity is exactly equal and
42 ELECTROSTATIC PHENOMENA. [40.
opposite to that of positive electricity, each of the m positive units
in A will attract each of the n' negative units in B with the same
force/*, making a total effect equal to mn'f.
Similarly the n negative units in A will attract the mf positive
units in B with a force nm'f, and will repel the n' negative units
in B with a force nn'f.
The total repulsion will therefore be (mm' + nn*)f\ and the total
attraction will he (mn'+ m'n}f.
The resultant repulsion will be
(mm' -f nn' — mn' — nm')f or (m — n) (mf — n')f.
Now m — n = e is the algebraical value of the charge on A, and
m'—n'=. ef is that of the charge on B, so that the resultant re
pulsion may be written eeff, the quantities e and e' being always
understood to be taken with their proper signs.
Variation of the Force with the Distance.
40.] Having established the law of force at a fixed distance,
we may measure the force between bodies charged in a constant
manner and placed at different distances. It is found by direct
measurement that the force, whether of attraction or repulsion,
varies inversely as the square of the distance, so that if f is the
repulsion between two units at unit distance, the repulsion at dis
tance r will be/1/*"2, and the general expression for the repulsion
between e units and e' units at distance r will be
Definition of the Electrostatic Unit of Electricity .
41.] We have hitherto used a wholly arbitrary standard for our
unit of electricity, namely, the electrification of a certain piece of
glass as it happened to be electrified at the commencement of our
experiments. We are now able to select a unit on a definite prin
ciple, and in order that this unit may belong to a general system
we define it so that/1 may be unity, or in other words —
The electrostatic unit of electricity is that quantity of electricity
which, when placed at unit of distance from an equal quantity -, repels
it with unit of force.
This unit is called the Electrostatic- unit to distinguish it from
the Electromagnetic unit, to be afterwards defined.
We may now write the general law of electrical action in the
simple form F = ee' r~* -, or,
43-] LAW OF ELECTRIC FORCE. 43
The repulsion between two small bodies charged respectively with, e and
e units of electricity is numerically equal to the product of the charges
divided by the square of the distance.
Dimensions of the Electrostatic Unit of Quantity.
42.] If [Q] is the concrete electrostatic unit of quantity itself,
and e> e' the numerical values of particular quantities ; if [Z] is
the unit of length, and r the numerical value of the distance ; and
if \F~\ is the unit of force, and F the numerical value of the force,
then the equation becomes .
whence [Q] =
This unit is called the Electrostatic Unit of electricity. Other
units may be employed for practical purposes, and in other depart
ments of electrical science, but in the equations of electrostatics
quantities of electricity are understood to be estimated in electro
static units, just as in physical astronomy we employ a unit of
mass which is founded on the phenomena of gravitation, and which
differs from the units of mass in common use.
Proof of the I/aw of Electrical Force.
43.] The experiments of Coulomb with the torsion-balance may
be considered to have established the law of force with a certain
approximation to accuracy. Experiments of this kind, however,
are rendered difficult, and in some degree uncertain, by several
disturbing causes, which must be carefully traced and corrected for.
In the first place, the two electrified bodies must be of sensible
dimensions relative to the distance between them, in order to be
capable of carrying charges sufficient to produce measurable forces.
The action of each body will then produce an effect on the dis
tribution of electricity on the other, so that the charge cannot be
considered as evenly distributed over the surface, or collected at
the centre of gravity ; but its effect must be calculated by an
intricate investigation. This, however, has been done as regards
two spheres by Poisson in an extremely able manner, and the
investigation has been greatly simplified by Sir W. Thomson in
his Theory of Electrical Images. See Arts. 172-174.
Another difficulty arises from the action of the electricity
induced on the sides of the case containing the instrument. By
4:4: ELECTROSTATIC PHENOMENA. [44.
making the inside of the instrument accurately cylindric, and
making- its inner surface of metal, this effect can be rendered
definite and measurable.
An independent difficulty arises from the imperfect insulation
of the bodies, on account of which the charge continually de
creases. Coulomb investigated the law of dissipation, and made
corrections for it in his experiments.
The methods of insulating charged conductors, and of measuring
electrical effects, have been greatly improved since the time of
Coulomb, particularly by Sir W. Thomson; but the perfect ac
curacy of Coulomb's law of force is established, not by any direct
experiments and measurements (which may be used as illustrations
of the law), but by a mathematical consideration of the pheno
menon described as Experiment VII, namely, that an electrified
conductor B, if made to touch the inside of a hollow closed con
ductor C and then withdrawn without touching C, is perfectly dis
charged, in whatever manner the outside of C may be electrified.
By means of delicate electroscopes it is easy to shew that no
electricity remains on B after the operation, and by the mathe
matical theory given at Art. 74, this can only be the case if the
force varies inversely as the square of the distance, for if the law
had been of any different form B would have been electrified.
The Electric Field.
44.] The Electric Field is the portion of space in the neigh
bourhood of electrified bodies, considered with reference to electric
phenomena. It may be occupied by air or other bodies, or it
may be a so-called vacuum, from which we have withdrawn every
substance which we can act upon with the means at our dis
posal.
If an electrified body be placed at any part of the electric field
it will be acted on by a force which will depend, in general, on
the shape of the body and on its charge, if the body is so highly
charged as to produce a sensible disturbance in the previous elec
trification of the other bodies.
But if the body is very small and its charge also very small,
the electrification of the other bodies will not be sensibly disturbed,
and we may consider the body as indicating by its centre of gravity
a certain point of the field. The force acting on the body will
then be proportional to its charge, and will be reversed when the
charge is reversed.
46.] ELECTRIC POTENTIAL. 45
Let e be the charge of the body, and F the force acting on the
body in a certain direction, then when e is very small F is propor
tional to e, or F = Re
where R is a quantity depending on the other bodies in the field.
If the charge e could be made equal to unity without disturbing
the electrification of other bodies we should have F = R.
We shall call R the Resultant electric force at the given point
of the field.
Electric Potential.
45.] If the small body carrying the small charge e be moved
from the given point to an indefinite distance from the electrified
bodies, it will experience at each point of its course a force Re,
where R varies from point to point of the course. Let the whole
work done on the body by these electrical forces be Ve, then V is
the potential at the point of the field from which the body started.
If the charge e could be made equal to unity without disturbing
the electrification of other bodies, we might define the potential at
any point as the work done on a body charged with unit of elec
tricity in moving from that point to an infinite distance.
A body electrified positively tends to move from places of greater
positive potential to places of smaller positive, or of negative
potential, and a body negatively electrified tends to move in the
opposite direction.
In a conductor the electrification is distributed exactly as if
it were free to move in the conductor according to the same law.
If therefore two parts of a conductor have different potentials,
positive electricity will move from the part having greater potential
to the part having less potential as long as that difference con
tinues. A conductor therefore cannot be in electrical equilibrium
unless every point in it has the same potential. This potential is
called the Potential of the Conductor.
Equipotential Surfaces.
46.] If a surface described or supposed to be described in the
electric field is such that the electric potential is the same at every
point of the surface it is called an Equipotential surface.
An electrified point constrained to rest upon such a surface will
have no tendency to move from one part of the surface to another,
because the potential is the same at every point. An equipotential
surface is therefore a surface of equilibrium or a level surface.
4»3 ELECTROSTATIC PHENOMENA. [46.
The resultant force at any point of the surface is in the direction
of the normal to the surface, and the magnitude of the force is such
that the work done on an electrical unit in passing from the surface
rtothesmfecePis T-T',
No two equipotential surfaces having different potentials can
one another, because the same point cannot have more than
potential, but one equipotential surface may meet itself, and
this takes place at all points and lines of equilibrium.
Gftte surface of a conductor in electrical eonfibrium is necessarily
an equipotential nmfki,. If the electrification of the conductor is
the whole surface, then the potrntinl will diminish as
away from the surface on every side, and the conductor
will be surrounded by a series of surfaces of lower potential.
But if (»••& to ike action of external electrified bodies) some
legions of Ike conductor are electrified positively and others ne
gatively, the complete equipotential surface will consist of the
of the conductor itself together with a system of other
meeting the surface of the conductor in the lines which
divide the positive from the negative regions. These lines will
be lines of equilibrium, so that an electrified point placed on one
of these lines will experience no force in any direction.
Whem the surface of a conductor is electrified positively in some
parts and negatively in others, there most be some other electrified
body in the field besides itself. For if we allow a positively
electrified point, starting from a positively electrified part of the
surface, to vote always in the direction of the resultant force upon
it, the potential at the point will continually diminish till the point
reaches either a negatively electrified surface at a potential less than
that of the first conductor, or uaeiei off to an infinite distance.
Since the fiiiBBiJal at an infinite distance is zero, the latter case
can only occur when the potential of the conductor is positive.
In the same way a negatively electrified point, moving off from
a negatively electrified pert of the surface, must either reach a posi
tively electrified surface, or pass off to infinity, and the latter case
can only happen when the potential of the conductor is negative.
Therefore, if both positive and negative electrification exists on
a conductor, there must be some other body in the field whose
potential has the same sign as that of the conductor but a greater
numerical value, and if a conductor of any form is alone in the
field the electrification of every part is of the same sign as the
potential of the conductor.
49-] ELECTBIC TENSION. 47
Line* of Force.
47.] The line described by a point moving- always in the direc
tion of the resultant force is called a Line of force. It cuts the
equipotential surfaces at right angles. The properties of lines of
force will be more fully explained afterwards, because Faraday has
expressed many of the laws of electrical action in terms of his
conception of lines of force drawn in the electric field, and indicating
both the direction and the magnitude of the force at every point.
Efectrlc Tension.
48.] Since the surface of a conductor is an equipotential surface,
the resultant force is normal to the surface, and it will be shewn
in Art. 78 that it is proportional to the superficial density of the
electrification. Hence the electricity on any small area of the
surface will be acted on by a force tending from the conductor
and proportional to the product of the resultant force and the
density, that is, proportional to the square of the resultant force
This force which acts outwards as a tension on every part of
the conductor will be called electric Tension. It is measured like
ordinary mechanical tension, by the force exerted on unit of area.
The word Tension has been used by electricians in several vague
senses, and it has been attempted to adopt it in mathematical
language as a synonym for Potential ; but on examining the cases
in which the word has been used, I think it will be more con
sistent with usage and with mechanical analogy to understand
by tension a pulling force of so many pounds per square inch
exerted on the surface of a conductor or elsewhere. We shall find
that the conception of Faraday, that this electric tension exists not
only at the electrified surface but all along the lines of force, leads
to a theory of electric action as a phenomenon of stress in a
medium.
Bbctrowotitt Force.
49.] When two conductors at different potentials are connected
by a thin conducting wire, the tendency of electricity to flow
along the wire is measured by the difference of the potentials of
the two bodies. The difference of potentials between two con
ductors or two points is therefore called the Electromotive force
between them.
Electromotive force mav arise from other causes than difference
48 ELECTROSTATIC PHENOMENA. [50.
of potential, but these causes are not considered in treating of sta
tical electricity. We shall consider them when we come to chemical
actions, motions of magnets, inequalities of temperature, &c.
Capacity of a Conductor.
50.] If one conductor is insulated while all the surrounding con
ductors are kept at the zero potential by being put in commu
nication with the earth, and if the conductor, when charged with
a quantity E of electricity, has a potential F, the ratio of E to V
is called the Capacity of the conductor. If the conductor is com
pletely enclosed within a conducting vessel without touching it,
then the charge on the inner conductor will be equal and op
posite to the charge on the inner surface of the outer conductor,
and will be equal to the capacity of the inner conductor multiplied
by the difference of the potentials of the two conductors.
Electric Accumulators.
A system consisting of two conductors whose opposed surfaces
are separated from each other by a thin stratum of an insulating
medium is called an electric Accumulator. Its capacity is directly
proportional to the area of the opposed surfaces and inversely pro
portional to the thickness of the stratum between them. A Leyden
jar is an accumulator in which glass is the insulating medium.
Accumulators are sometimes called Condensers, but I prefer to
restrict the term ' condenser' to an instrument which is used not to
hold electricity but to increase its superficial density.
PROPERTIES OF BODIES IN RELATION TO STATICAL ELECTRICITY.
Resistance to the Passage of Electricity through a Body.
51.] When a charge of electricity is communicated to any part
of a mass of metal the electricity is rapidly transferred from places
of high to places of low potential till the potential of the whole
mass becomes the same. In the case of pieces of metal used in
ordinary experiments this process is completed in a time too short
to be observed, but in the case of very long and thin wires, such
as those used in telegraphs, the potential does not become uniform
till after a sensible time, on account of the resistance of the wire
to the passage of electricity through it.
The resistance to the passage of electricity is exceedingly dif
ferent in different substances, as may be seen from the tables at
51.] ELECTRIC RESISTANCE. 49
Arts. 362, 366, and 369, which will be explained in treating of
Electric Currents.
All the metals are good conductors, though the resistance of
lead is 12 times that of copper or silver, that of iron 6 times,
and that of mercury 60 times that of copper. The resistance of all
metals increases as their temperature rises.
Selenium in its crystalline state may also he regarded as a con
ductor, though its resistance is 3.7 x 1012 times that of a piece
of copper of the same dimensions. Its resistance increases as the
temperature rises. Selenium in the amorphous form is a good
insulator, like sulphur.
Many liquids conduct electricity by electrolysis. This mode of
conduction will be considered in Part II. For the present, we may
regard all liquids containing water and all damp bodies as con
ductors, far inferior to the metals, but incapable of insulating a
charge of electricity for a sufficient time to be observed.
On the other hand, the gases at the atmospheric pressure, whether
dry or moist, are insulators so nearly perfect when the electric tension
is small that we have as yet obtained no evidence of electricity passing
through them by ordinary conduction. The gradual loss of charge
by electrified bodies may in every case be traced to imperfect insu
lation in the supports, the electricity either passing through the
substance of the support or creeping over its surface. Hence, when
two charged bodies are hung up near each other, they will preserve
their charges longer if they are electrified in opposite ways, than if
they are electrified in the same way. For though the electromotive
force tending to make the electricity pass through the air between
them is much greater when they are oppositely electrified, no per
ceptible loss occurs in this way. The actual loss takes place through
the supports, and the electromotive force through the supports is
greatest when the bodies are electrified in the same way. The result
appears anomalous only when we expect the loss to occur by the
passage of electricity through the air between the bodies.
Certain kinds of glass when cold are marvelously perfect in
sulators, and Sir W. Thomson has preserved charges of electricity
for years in bulbs hermetically sealed. The same glass, however,
becomes a conductor at a temperature below that of boiling water.
Gutta-percha, caoutchouc, vulcanite, paraffin, and resins are good
insulators, the resistance of gutta-percha at 75°F. being about
6 x 1019 times that of copper.
Ice, crystals, and solidified electrolytes, are also insulators.
VOL. I. E
50 ELECTROSTATIC PHENOMENA. [52.
Certain liquids, such as naphtha, turpentine, and some oils, are
insulators, but inferior to most of the solid insulators.
The resistance of most substances, except the metals, and selenium
and carbon, seems to diminish as the temperature rises.
DIELECTRICS.
Specific Inductive Capacity.
52.] All bodies whose insulating power is such that when they
are placed between two conductors at different potentials the elec
tromotive force acting on them does not immediately distribute
their electricity so as to reduce the potential to a constant value, are
called by Faraday Dielectrics.
Faraday discovered that the capacity of an accumulator depends
on the nature of the insulating medium between the two conductors,
as well as on the dimensions and relative position of the conductors
themselves. By substituting other insulating media for air as the
dielectric of the accumulator, without altering it in any other
respect, he found that when air and other gases were employed as
the insulating medium the capacity of the accumulator remained the
same, but that when shell-lac, sulphur, glass, &c., were substituted
for air, the capacity was increased in a ratio which was different
for each substance.
The ratio of the capacity of an accumulator formed of any di
electric medium to the capacity of an accumulator of the same form
and dimensions filled with air, was named by Faraday the Specific
Inductive Capacity of the dielectric medium. It is equal to unity
for air and other gases at all pressures, and probably at all tempe
ratures, and it is greater than unity for all other liquid or solid
dielectrics which have been examined.
If the dielectric is not a good insulator, it is difficult to mea
sure its inductive capacity, because the accumulator will not hold a
charge for a sufficient time to allow it to be measured ; but it is
certain that inductive capacity is a property not confined to good
insulators, and it is probable that it exists in all bodies.
Absorption of Electricity.
53.] It is found that when an accumulator is formed of certain
dielectrics, the following phenomena occur.
When the accumulator has been for some time electrified and is
then suddenly discharged and again insulated, it becomes recharged
54-] ELECTRIC ABSORPTION. 51
in the same sense as at first, but to a smaller degree, so that it may
be discharged again several times in succession, these discharges
always diminishing. This phenomenon is called that of the Re
sidual Discharge.
The instantaneous discharge appears always to be proportional
to the difference of potentials at the instant of discharge, and the
ratio of these quantities is the true capacity of the accumulator;
but if the contact of the discharger is prolonged so as to include
some of the residual discharge, the apparent capacity of the accu
mulator, calculated from such a discharge, will be too great.
The accumulator if charged and left insulated appears to lose its
charge by conduction, but it is found that the proportionate rate
of loss is much greater at first than it is afterwards, so that the
measure of conductivity, if deduced from what takes place at first,
would be too great. Thus, when the insulation of a submarine
cable is tested, the insulation appears to improve as the electrifi
cation continues.
Thermal phenomena of a kind at first sight analogous take place
in the case of the conduction of heat when the opposite sides of a
body are kept at different temperatures. In the case of heat we
know that they depend on the heat taken in and given out by the
body itself. Hence, in the case of the electrical phenomena, it
has been supposed that electricity is absorbed and emitted by the
parts of the body. We shall see, however, in Art. 329, that the
phenomena can be explained without the hypothesis of absorption of
electricity, by supposing the dielectric in some degree heterogeneous.
That the phenomenon called Electric Absorption is not an
actual absorption of electricity by the substance may be shewn by
charging the substance in any manner with electricity while it is
surrounded by a closed metallic insulated vessel. If, when the
substance is charged and insulated, the vessel be instantaneously
discharged and then left insulated, no charge is ever communicated
to the vessel by the gradual dissipation of the electrification of the
charged substance within it.
54.] This fact is expressed by the statement of Faraday that
it is impossible to charge matter with an absolute and independent
charge of one kind of electricity *.
In fact it appears from the result of every experiment which
has been tried that in whatever way electrical actions may take
* Exp. Res., vol. i. series xi. f ii. 'On the Absolute Charge of Matter,' and (1244).
E 2
52 ELECTROSTATIC PHENOMENA. [55.
place among a system of bodies surrounded by a metallic vessel, the
charge on the outside of that vessel is not altered.
Now if any portion of electricity could be forced into a body
so as to be absorbed in it, or to become latent, or in any way
to exist in it, without being connected with an -equal portion of
the opposite electricity by lines of induction, or if, after having
being absorbed, it could gradually emerge and return to its or
dinary mode of action, we should find some change of electrifica
tion in the surrounding vessel.
As this is never found to be the case, Faraday concluded that
it is impossible to communicate an absolute charge to matter, and
that no portion of matter can by any change of state evolve or
render latent one kind of electricity or the other. He therefore
regarded induction as ' the essential function both in the first
development and the consequent phenomena of electricity/ His
* induction' is (1298) a polarized state of the particles of the
dielectric, each particle being positive on one side and negative
on the other, the positive and the negative electrification of each
particle being always exactly equal.
Disruptive Discharge *.
55.] If the electromotive force acting at any point of a dielectric
is gradually increased, a limit is at length reached at which there
is a sudden electrical discharge through the dielectric, generally
accompanied with light and sound, and with a temporary or per
manent rupture of the dielectric.
The intensity of the electromotive force when this takes place
depends on the nature of the dielectric. It is greater, for instance,
in dense air than in rare air, and greater in glass than in air, but
in every case, if the electromotive force be made great enough,
the dielectric gives way and its insulating power is destroyed, so
that a current of electricity takes place through it. It is for this
reason that distributions of electricity for which the electric resultant
force becomes anywhere infinite cannot exist in nature.
The Electric Glow.
Thus, when a conductor having a sharp point is electrified,
the theory, based on the hypothesis that it retains its charge,
leads to the conclusion that as we approach the point the super
ficial density of the electricity increases without limit, so that at
the point itself the surface-density, and therefore the resultant
* See Faraday, Exp. Rts., vol. i., series xii. and xiii.
55-] ELECTRIC GLOW. 53
electrical force, would be infinite. If the air, or other surrounding
dielectric, had an invincible insulating power, this result would
actually occur ; but the fact is, that as soon as the resultant force
in the neighbourhood of the point has reached a certain limit, the
insulating power of the air gives way, so that the air close to
the point becomes a conductor. At a certain distance from the
point the resultant force is not sufficient to break through the
insulation of the air, so that the electric current is checked, and
the electricity accumulates in the air round the point.
The point is thus surrounded by particles of air charged with
electricity of the same kind with its own. The effect of this charged
air round the point is to relieve the air at the point itself from
part of the enormous electromotive force which it would have ex
perienced if the conductor alone had been electrified. In fact the
surface of the electrified body is no longer pointed, because the
point is enveloped by a rounded mass of electrified air, the surface
of which, rather than that of the solid conductor, may be regarded
as the outer electrified surface.
If this portion of electrified air could be kept still, the elec
trified body would retain its charge, if not on itself at least in its
neighbourhood, but the charged particles of air being free to move
under the action of electrical force, tend to move away from the elec
trified body because it is charged with the same kind of electricity.
The charged particles of air therefore tend to move off in the direc
tion of the lines of force and to approach those surrounding bodies
which are oppositely electrified. When they are gone, other un
charged particles take their place round the point, and since these
cannot shield those next the point itself from the excessive elec
tric tension, a new discharge takes place, after which the newly
charged particles move off, and so on as long as the body remains
electrified.
In this way the following phenomena are produced : — At and
close to the point there is a steady glow, arising from the con
stant discharges which are taking place between the point and the
air very near it.
The charged particles of air tend to move off in the same general
direction, and thus produce a current of air from the point, con
sisting of the charged particles, and probably of others carried along
by them. By artificially aiding this current we may increase the
glow, and by checking the formation of the current we may pre
vent the continuance of the glow.
54 ELECTROSTATIC PHENOMENA. [56.
The electric wind in the neighbourhood of the point is sometimes
very rapid, but it soon loses its velocity, and the air with its charged
particles is carried about with the general motions of the atmo
sphere, and constitutes an invisible electric cloud. When the charged
particles come near to any conducting surface, such as a wall, they
induce on that surface an electrification opposite to their own, and
are then attracted towards the wall, but since the electromotive
force is small they may remain for a long time near the wall
without being drawn up to the surface and discharged. They
thus form an electrified atmosphere clinging to conductors, the pre
sence of which may sometimes be detected by the electrometer.
The electrical forces, however, acting between charged portions
of air and other bodies are exceedingly feeble compared with the
forces which produce winds arising from inequalities of density
due to differences of temperature, so that it is very improbable
that any observable part of the motion of ordinary thunder clouds
arises from electrical causes.
The passage of electricity from one place to another by the
motion of charged particles is called Electrical Convection or Con-
vective Discharge.
The electrical glow is therefore produced by the constant passage
of electricity through a small portion of air in which the tension
is very high, so as to charge the surrounding particles of air which
are continually swept off by the electric wind, which is an essential
part of the phenomenon.
The glow is more easily formed in rare air than in dense air,
and more easily when the point is positive than when it is negative.
This and many other differences between positive and negative elec
trification must be studied by those who desire to discover some
thing about the nature of electricity. They have not, however,
been satisfactorily brought to bear upon any existing theory.
The Electric Brush.
56.] The electric brush is a phenomenon which may be pro
duced by electrifying a blunt point or small ball so as to produce
an electric field in which the tension diminishes, but in a less, rapid
manner, as we leave the surface. It consists of a succession of
discharges, ramifying as they diverge from the ball into the air,
and terminating either by charging portions of air or by reaching
some other conductor. It is accompanied by a sound, the pitch of
which depends on the interval between the successive discharges,
and there is no current of air as in the case of the glow.
57-] ELECTRIC SPARK. 55
The Electric Spark.
57.] When the tension in the space between two conductors is
considerable all the way between them, as in the case of two balls
whose distance is not great compared with their radii, the discharge,
when it occurs, usually takes the form of a spark, by which nearly
the whole electrification is discharged at once.
In this case, when any part of the dielectric has given way,
the parts on either side of it in the direction of the electric force
are put into a state of greater tension so that they also give way,
and so the discharge proceeds right through the dielectric, just as
when a little rent is made in the edge of a piece of paper a tension
applied to the paper in the direction of the edge causes the paper to
be torn through, beginning at the rent, but diverging occasionally
where there are weak places in the paper. The electric spark in
the same way begins at the point where the electric tension first
overcomes the insulation of the dielectric, and proceeds from that
point, in an apparently irregular path, so as to take in other weak
points, such as particles of dust floating in air.
0)i the Electric Force required to produce a Spark in Air.
In the experiments of Sir W. Thomson * the electromotive force
required to produce a spark across strata of air of various thick
nesses was measured by means of an electrometer.
The sparks were made to pass between two surfaces, one of which
was plane, and the other only sufficiently convex to make the sparks
occur always at the same place.
The difference of potential required to cause a spark to pass was
found to increase with the distance, but in a less rapid ratio, so that
the electric force at any point between the surfaces, which is the
quotient of the difference of potential divided by the distance, can
be raised to a greater value without a discharge when the stratum
of air is thin.
When the stratum of air is very thin, say .00254 of a centimetre,
the resultant force required to produce a spark was found to be
527.7, in terms of centimetres and grammes. This corresponds to
an electric tension of 1 1.29 grammes weight per square centimetre.
When the distance between the surfaces is about a millimetre
the electric force is about 130, and the electric tension .68 grammes
weight per square centimetre. It is probable that the value for
* Proc. K. S., I860 ; or, Reprint, chap. xix.
56 ELECTROSTATIC PHENOMENA* [58.
greater distances is not much less than this. The ordinary pressure
of the atmosphere is about 1032 grammes per square centimetre.
It is difficult to explain why a thin stratum of air should require
a greater force to produce a disruptive discharge across it than a
thicker stratum. Is it possible that the air very near to the sur
face of dense bodies is condensed, so as to become a better insu
lator ? or does the potential of an electrified conductor differ from
that of the air in contact with it by a quantity having a maximum
value just before discharge, so that the observed difference of
potential of the conductors is in every case greater than the dif
ference of potentials on the two sides of the stratum of air by a
constant quantity equivalent to the addition of about .005 of an
inch to the thickness of the stratum ? See Art. 370.
All these phenomena differ considerably in different gases, and in
the same gas at different densities. Some of the forms of electrical
discharge through rare gases are exceedingly remarkable. In some
cases there is a regular alternation of luminous and dark strata, so
that if the electricity, for example, is passing along a tube contain
ing a very small quantity of gas, a number of luminous disks will
be seen arranged transversely at nearly equal intervals along the
axis of the tube and separated by dark strata. If the strength of
the current be increased a new disk will start into existence, and
it and the old disks will arrange themselves in closer order. In
a tube described by Mr. Gassiot"* the light of each of the disks
is bluish on the negative and reddish on the positive side, and
bright red in the central stratum.
These, and many other phenomena of electrical discharge, are
exceedingly important, and when they are better understood they
will probably throw great light on the nature of electricity as well
as on the nature of gases and of the medium pervading space. At
present, however, they must be considered as outside the domain of
the mathematical theory of electricity.
Electric Phenomena of Tourmaline.
58.] Certain crystals of tourmaline, and of other minerals, possess
what may be called Electric Polarity. Suppose a crystal of tour
maline to be at a uniform temperature, and apparently free from
electrification on its surface. Let its temperature be now raised,
the crystal remaining insulated. One end will be found positively
* Intellectual Observer, March, 1866.
59-] ELECTRIFICATION OF TOURMALINE. 57
and the other end negatively electrified. Let the surface be de
prived of this apparent electrification by means of a flame or other
wise, then if the crystal be made still hotter, electrification of the
same kind as before will appear, but if the crystal be cooled the
end which was positive when the crystal was heated will become
negative.
These electrifications are observed at the extremities of the crys-
tallographic axis. Some crystals are terminated by a six-sided
pyramid at one end and by a three- sided pyramid at the other.
In these the end having the six-sided pyramid becomes positive
when the crystal is heated.
Sir W. Thomson supposes every portion of these and other hemi-
hedral crystals to have a definite electric polarity, the intensity
of which depends on the temperature. When the surface is passed
through a flame, every part of the surface becomes electrified to
such an extent as to exactly neutralize, for all external points,
the effect of the internal polarity. The crystal then has no ex
ternal electrical action, nor any tendency to change its mode of
electrification. But if it be heated or cooled the interior polariza
tion of each particle of the crystal is altered, and can no longer
be balanced by the superficial electrification, so that there is a
resultant external action.
Plan of this Treatise.
59.] In the following treatise I propose first to explain the ordinary
theory of electrical action, which considers it as depending only
on the electrified bodies and on their relative position, without
taking account of any phenomena which may take place in the
surrounding media. In this way we shall establish the law of the
inverse square, the theory of the potential, and the equations of
Laplace and Poisson. We shall next consider the charges and
potentials of a system of electrified conductors as connected by
a system of equations, the coefficients of which may be supposed
to be determined by experiment in those cases in which our present
mathematical methods are not applicable, and from these we shall
determine the mechanical forces acting between the different elec
trified bodies.
We shall then investigate certain general theorems by which
Green, Gauss, and Thomson have indicated the conditions of so
lution of problems in the distribution of electricity. One result
of these theorems is, that if Poisson's equation is satisfied by any
58 ELECTROSTATIC PHENOMENA. [59.
function, and if at the surface of every conductor the function
has the value of the potential of that conductor, then the func
tion expresses the actual potential of the system at every point. We
also deduce a method of finding problems capable of exact solution.
In Thomson's theorem, the total energy of the system is ex
pressed in the form of the integral of a certain quantity extended
over the whole space between the electrified bodies, and also in
the form of an integral extended over the electrified surfaces only.
The equation between these two expressions may be thus inter
preted physically. We may conceive the relation into which the
electrified bodies are thrown, either as the result of the state of
the intervening medium, or as the result of a direct action between
the electrified bodies at a distance. If we adopt the latter con
ception, we may determine the law of the action, but we can go
no further in speculating on its cause. If, on the other hand,
we adopt the conception of action through a medium, we are led to
enquire into the nature of that action in each part of the medium.
It appears from the theorem, that if we are to look for the seat
of the electric energy in the different parts of the dielectric me
dium, the amount of energy in any small part must depend on
the square of the intensity of the resultant electromotive force at
that place multiplied by a coefficient called the specific inductive
capacity of the medium.
It is better, however, in considering the theory of dielectrics
in the most general point of view, to distinguish between the elec
tromotive force at any point and the electric polarization of the
medium at that point, since these directed quantities, though re
lated to one another, are not, in some solid substances, in the same
direction. The most general expression for the electric energy of
the medium per unit of volume is half the product of the electro
motive force and the electric polarization multiplied by the cosine
of the angle between their directions.
In all fluid dielectrics the electromotive force and the electric
polarization are in the same direction and in a constant ratio.
If we calculate on this hypothesis the total energy residing
in the medium, we shall find it equal to the energy due to the
electrification of the conductors on the hypothesis of direct action
at a distance. Hence the two hypotheses are mathematically equi
valent.
If we now proceed to investigate the mechanical state of the
medium on the hypothesis that the mechanical action observed
6O.] STRESS IN DIELECTRICS. 59
between electrified bodies is exerted through and by means of
the medium, as in the familiar instances of the action of one body
on another by means of the tension of a rope or the pressure of
a rod, we find that the medium must be in a state of mechanical
stress.
The nature of this stress is, as Faraday pointed out *, a tension
along the lines of force combined with an equal pressure in all
directions at right angles to these lines. The magnitude of these
stresses is proportional to the energy of the electrification, or, in
other words, to the square of the resultant electromotive force mul
tiplied by the specific inductive capacity of the medium.
This distribution of stress is the only one consistent with the
observed mechanical action on the electrified bodies, and also with
the observed equilibrium of the fluid dielectric which surrounds
them. I have therefore thought it a warrantable step in scientific
procedure to assume the actual existence of this state of stress, and
to follow the assumption into its consequences. Finding the phrase
electric tension used in several vague senses, I have attempted to
confine it to what I conceive to have been in the mind of some
of those who have used it, namely, the state of stress in the
dielectric medium which causes motion of the electrified bodies,
and leads, when continually augmented, to disruptive discharge.
Electric tension, in this sense, is a tension of exactly the same
kind, and measured in the same way, as the tension of a rope,
and the dielectric medium, which can support a certain tension
and no more, may be said to have a certain strength in exactly
the same sense as the rope is said to have a certain strength.
Thus, for example, Thomson has found that air at the ordinary
pressure and temperature can support an electric tension of 9600
grains weight per square foot before a spark passes.
60.] From the hypothesis that electric action is not a direct
action between bodies at a distance, but is exerted by means of
the medium between the bodies, we have deduced that this medium
must be in a state of stress. We have also ascertained the cha
racter of the stress, and compared it with the stresses which may
occur in solid bodies. Along the lines of force there is tension,
and perpendicular to them there is pressure, the numerical mag
nitude of these forces being equal, and each proportional to the
square of the resultant force at the point. Having established
these results, we are prepared to take another step, and to form
* Exp. Bes., series xi. 1297.
60 ELECTROSTATIC PHENOMENA. [60.
an idea of the nature of the electric polarization of the dielectric
medium.
An elementary portion of a body may be said to be polarized
when it acquires equal and opposite properties on two opposite
sides. The idea of internal polarity may be studied to the greatest
advantage as exemplified in permanent magnets, and it will be
explained at greater length when we come to treat of magnetism.
The electric polarization of an elementary portion of a dielectric
is a forced state into which the medium is thrown by the action
of electromotive force, and which disappears when that force is
removed. We may conceive it to consist in what we may call
an electrical displacement, produced by the electromotive force.
When the electromotive force acts on a conducting medium it
produces a current through it, but if the medium is a non-con
ductor or dielectric, the current cannot flow through the medium,
but the electricity is displaced within the medium in the direction
of the electromotive force, the extent of this displacement de
pending on the magnitude of the electromotive force, so that if
the electromotive force increases or diminishes the electric displace
ment increases and diminishes in the same ratio.
The amount of the displacement is measured by the quantity
of electricity which crosses unit of area, while the displacement
increases from zero to its actual amount. This, therefore, is the
measure of the electric polarization.
The analogy between the action of electromotive force in pro
ducing electric displacement and of ordinary mechanical force in
producing the displacement of an elastic body is so obvious that
I have ventured to call the ratio of the electromotive force to the
corresponding electric displacement the coefficient of electric elasticity
of the medium. This coefficient is different in different media, and
varies inversely as the specific inductive capacity of each medium.
The variations of electric displacement evidently constitute electric
currents. These currents, however, can only exist during the
variation of the displacement, and therefore, since the displace
ment cannot exceed a certain value without causing disruptive
discharge, they cannot be continued indefinitely in the same direc
tion, like the currents through conductors.
In tourmaline, and other pyro-electric crystals, it is probable that
a state of electric polarization exists, which depends upon tem
perature, and does not require an external electromotive force to
produce it If the interior of a body were in a state of permanent
60.] ELECTRIC DISPLACEMENT. 61
electric polarization, the outside would gradually become charged
in such a manner as to neutralize the action of the internal elec
trification for all points outside the body. This external superficial
charge could not be detected by any of the ordinary tests, and
could not be removed by any of the ordinary methods for dis
charging superficial electrification. The internal polarization of
the substance would therefore never be discovered unless by some
means, such as change of temperature, the amount of the internal
polarization could be increased or diminished. The external elec
trification would then be no longer capable of neutralizing the
external effect of the internal polarization, and an apparent elec
trification would be observed, as in the case of tourmaline.
If a charge e is uniformly distributed over the surface of a
sphere, the resultant force at any point of the medium surrounding
the sphere is numerically equal to the charge e divided by the square
of the distance from the centre of the sphere. This resultant force,
according to our theory, is accompanied by a displacement of elec
tricity in a direction outwards from the sphere.
If we now draw a concentric spherical surface of radius r , the whole
displacement, E, through this surface will be proportional to the
resultant force multiplied by the area of the spherical surface. But
the resultant force is directly as the charge e and inversely as the
square of the radius, while the area of the surface is directly as the
square of the radius.
Hence the whole displacement, E, is proportional to the charge e,
and is independent of the radius.
To determine the ratio between the charge £, and the quantity
of electricity, E, displaced outwards through the spherical surface,
let us consider the work done upon the medium in the region
between two concentric spherical surfaces, while the displacement
is increased from E to E+bE. If V^ and Y2 denote the potentials
at the inner and the outer of these surfaces respectively, the elec
tromotive force by which the additional displacement is produced
is Vl — F2, so that the work spent in augmenting the displacement
If we now make the inner surface coincide with that of the
electrified sphere, and make the radius of the other infinite, V^
becomes F, the potential of the sphere, and V.> becomes zero, so
that the whole work done in the surrounding medium is T§E.
But by the ordinary theory, the work done in augmenting the
charge is Fbe, and if this is spent, as we suppose, in augmenting
62 ELECTEOSTATIC PHENOMENA. [6 1.
the displacement, bU = be, and since E and e vanish together,
E—e, or —
The displacement outwards through any spherical surface concentric
with the sphere is equal to the charge on the sphere.
To fix our ideas of electric displacement, let us consider an accu
mulator formed of two conducting plates A and j5, separated by a
stratum of a dielectric C. Let W be a conducting wire joining
A and JB, and let us suppose that by the action of an electromotive
force a quantity Q of positive electricity is transferred along the
wire from B to A. The positive electrification of A and the
negative electrification of £ will produce a certain electromotive
force acting from A towards £ in the dielectric stratum, and this
will produce an electric displacement from A towards JB within the
dielectric. The amount of this displacement, as measured by the
quantity of electricity forced across an imaginary section of the
dielectric dividing it into two strata, will be, according to our
theory, exactly Q. See Arts. 75, 76, 111.
It appears, therefore, that at the same time that a quantity
Q of electricity is being transferred along the wire by the electro
motive force from B towards A, so as to cross every section of
the wire, the same quantity of electricity crosses every section
of the dielectric from A towards B by reason of the electric dis
placement.
The reverse motions of electricity will take place during the
discharge of the accumulator. In the wire the discharge will be
Q from A to B, and in the dielectric the displacement will subside,
and a quantity of electricity Q will cross every section from B
towards A.
Every case of electrification or discharge may therefore be con
sidered as a motion in a closed circuit, such that at every section
of the circuit the same quantity of electricity crosses in the same
time, and this is me case, not only in the voltaic circuit where
it has always been recognised, but in those cases in which elec
tricity has been generally supposed to be accumulated in certain
places.
61.] We are thus led to a very remarkable consequence of the
theory which we are examining, namely, that the motions of elec
tricity are like those of an incompressible fluid, so that the total
quantity within an imaginary fixed closed surface remains always
the same. This result appears at first sight in direct contradiction
to the fact that we can charge a conductor and then introduce
62.] THEORY PROPOSED. 63
it into the closed space, and so alter the quantity of electricity
within that space. But we must remember that the ordinary theory
takes no account of the electric displacement in the substance of
dielectrics which we have been investigating, but confines its
attention to the electrification at the bounding surfaces of the
conductors and dielectrics. In the case of the charged conductor
O
let us suppose the charge to be positive, then if the surrounding
dielectric extends on all sides beyond the closed surface there will be
electric polarization, accompanied with displacement from within
outwards all over the closed surface, and the surface-integral of
the displacement taken over the surface will be equal to the charge
on the conductor within.
Thus when the charged conductor is introduced into the closed
space there is immediately a displacement of a quantity of elec
tricity equal to the charge through the surface from within out
wards, and the whole quantity within the surface remains the
same.
The theory of electric polarization will be discussed at greater
length in Chapter V, and a mechanical illustration of it will be
given in Art. 334, but its importance cannot be fully understood
till we arrive at the study of electromagnetic phenomena.
62.] The peculiar features of the theory as we have now de
veloped them are : —
That the energy of electrification resides in the dielectric medium,
whether that medium be solid, liquid, or gaseous, dense or rare,
or even deprived of ordinary gross matter, provided it be still
capable of transmitting electrical action.
That the energy in any part of the medium is stored up in
the form of a state of constraint called electric polarization, the
amount of which depends on the resultant electromotive force at
the place. ^
That electromotive force acting on a diele^iicr produces what
we have called electric displacement, the relation between the force
and the displacement being in the most general case of a kind
to be afterwards investigated in treating of conduction, but in
the most important cases the force is in the same direction as
the displacement, and is numerically equal to the displacement
multiplied by a quantity which we have called the coefficient of
electric elasticity of the dielectric.
That the energy per unit of volume of the dielectric arising from
the electric polarization is half the product of the electromotive
64 ELECTROSTATIC PHENOMENA. [62.
force and the electric displacement multiplied, if necessary, by the
cosine of the angle between their directions.
That in fluid dielectrics the electric polarization is accompanied
by a tension in the direction of the lines of force combined with
an equal pressure in all directions at right angles to the lines
of force, the amount of the tension or pressure per unit of area
being numerically equal to the energy per unit of volume at the
same place.
That the surfaces of any elementary portion into which we may
conceive the volume of the dielectric divided must be conceived
to be electrified, so that the surface- density at any point of the
surface is equal in magnitude to the displacement through that
point of the surface reckoned inwards, so that if the displacement
is in the positive direction, the surface of the element will be elec
trified negatively on the positive side and positively on the negative
side. These superficial electrifications will in general destroy one
another when consecutive elements are considered, except where
the dielectric has an internal charge, or at the surface of the
dielectric.
That whatever electricity may be, and whatever we may under
stand by the movement of electricity, the phenomenon which we
have called electric displacement is a movement of electricity in the
same sense as the transference of a definite quantity of electricity
through a wire is a movement of electricity, the only difference
being that in the dielectric there is a force which we have called
electric elasticity which acts against the electric displacement, and
forces the electricity back when the electromotive force is removed ;
whereas in the conducting wire the electric elasticity is continually
giving way, so that a current of true conduction is set up, and
the resistance depends, not on the total quantity of electricity dis
placed from its position of equilibrium, but on the quantity which
crosses a section of the conductor in a given time.
That in every case the motion of electricity is subject to the
same condition as that of an incompressible fluid, namely, that
at every instant as much must flow out of any given closed space
as flows into it.
It follows from this that every electric current must form a
closed circuit. The importance of this result will be seen when we
investigate the laws of electro-magnetism.
Since, as we have seen, the theory of direct action at a distance
is mathematically identical with that of action by means of a
62.] METHOD OF THIS WORK. 65
medium, the actual phenomena may be explained by the one
theory as well as by the other, provided suitable hypotheses be
introduced when any difficulty occurs. Thus, Mossotti has deduced
the mathematical theory of dielectrics from the ordinary theoiy
of attraction by merely giving an electric instead of a magnetic
interpretation to the symbols in the investigation by which Poisson
has deduced the theory of magnetic induction from the theory of
magnetic fluids. He assumes the existence within the dielectric of
small conducting elements, capable of having their opposite surfaces
oppositely electrified by induction,, but not capable of losing or
gaining electricity on the whole, owing to their being insulated
from each other by a non-conducting medium. This theory of
dielectrics is consistent with the laws of electricity, and may be
actually true. If it is true, the specific inductive capacity of a
dielectric may be greater, but cannot be less, than that of air or
vacuum. No instance has yet been found of a dielectric having
an inductive capacity less than that of air, but if such should
be discovered, Mossotti's theory must be abandoned, although his
formulae would all remain exact, and would only require us to alter
the sign of a coefficient.
In the theory which I propose to develope, the mathematical
methods are founded upon the smallest possible amount of hypo
thesis, and thus equations of the same form are found applicable to
phenomena which are certainly of quite different natures, as, for
instance, electric induction through dielectrics ; conduction through
conductors, and magnetic induction. In all these cases the re
lation between the force and the effect produced is expressed by
a set of equations of the same kind, so that when a problem in
one of these subjects is solved, the problem and its solution may
be translated into the language of the other subjects and the
results in their new form will also be true.
VOL. I.
CHAPTER II.
%
ELEMENTARY MATHEMATICAL THEORY OF STATICAL
ELECTRICITY.
Definition of Electricity as a Mathematical Quantity.
63.] We have seen that the actions of electrified bodies are such
that the electrification of one body may be equal to that of another,
or to the sum of the electrifications of two bodies, and that when
two bodies are equally and oppositely electrified they have no elec
trical effect on external bodies when placed together within a closed
insulated conducting vessel. We may express all these results in
a concise and consistent manner by describing an electrified body as
charged with a certain quantity of electricity, which we may denote
by e. When the electrification is positive, that is, according to the
usual convention, vitreous, e will be a positive quantity. When the
electrification is negative or resinous, e will be negative, and the
quantity — e may be interpreted either as a negative quantity of
vitreous electricity or as a positive quantity of resinous electricity.
The effect of adding together two equal and opposite charges of
electricity, +e and — e, is to produce a state of no electrification
expressed by zero. We may therefore regard a body not electrified
as virtually charged with equal and opposite charges of indefinite
magnitude, and an electrified body as virtually charged with un
equal quantities of positive and negative electricity, the algebraic
sum of these charges constituting the observed electrification. It is
manifest, however, that this way of regarding an electrified body
is entirely artificial, and may be compared to the conception of the
velocity of a body as compounded of two or more different velo
cities, no one of which is the actual velocity of the body. When
we speak therefore of a body being charged with a quantity e of
electricity we mean simply that the body is electrified, and that
the electrification is vitreous or resinous according as e is positive
or negative.
64.] ELECTRIC DENSITY. 67
ON ELECTRIC DENSITY.
Distribution in Three Dimensions.
64.] Definition. The electric volume-density at a given point
in space is the limiting ratio of the quantity of electricity within
a sphere whose centre is the given point to the volume of the
sphere, when its radius is diminished without limit.
We shall denote this ratio by the symbol p, which may be posi
tive or negative.
Distribution on a Surface.
It is a result alike of theory and of experiment, that, in certain
cases, the electrification of a body is entirely on the surface. The
density at a point on the surface, if defined according to the method
given above, would be infinite. We therefore adopt a different
method for the measurement of surface-density.
Definition. The electric density at a given point on a surface is
the limiting ratio of the quantity of electricity within a sphere
whose centre is the given point to the area of the surface contained
within the sphere, when its radius is diminished without limit.
We shall denote the surface-density by the symbol <r.
Those writers who supposed electricity to be a material fluid
or a collection of particles, were obliged in this case to suppose
the electricity distributed on the surface in the form of a stratum
of a certain thickness 0, its density being p0, or that value of p
which would result from the particles having the closest contact
of which they are capable. It is manifest that on this theory
p0 6 = a:
When cr is negative, according to this theory, a certain stratum
of thickness 0 is left entirely devoid of positive electricity, and
filled entirely with negative electricity.
There is, however, no experimental evidence either of the elec
tric stratum having any thickness, or of electricity being a fluid
or a collection of particles. We therefore prefer to do without the
symbol for the thickness of the stratum, and to use a special symbol
for surface-density.
Distribution, along a Line.
It is sometimes convenient to suppose electricity distributed
on a line, that is, a long narrow body of which we neglect the
68 ELECTROSTATICS. [65.
thickness. In this case we may define the line-density at any point
to be the limiting1 ratio of the electricity on an element of the
line to the length of that element when the element is diminished
without limit.
If A denotes the line-density, then the whole quantity of elec
tricity on a curve is e = I A da, where ds is the element of the curve.
Similarly, if o- is the surface-density, the whole quantity of elec
tricity on the surface is
e —
where dS is the element of surface.
If p is the volume-density at any point of space, then the whole
electricity within a certain volume is
e = I I I p dx dy dz,
where dx dy dz is the element of volume. The limits of integration
in each case are those of the curve, the surface, or the portion of
space considered.
It is manifest that e, A, o- and p are quantities differing in kind,
each being one dimension in space lower than the preceding, so that
if a be a line, the quantities e, a\, a2 a; and a^p will be all of the
same kind, and if a be the unit of length, and A, o-, p each the
unit of the different kinds of density, a\, a2 a; and a3 p will each
denote one unit of electricity.
Definition of the Unit of Electricity.
65.] Let A and B be two points the distance between which
is the unit of length. Let two bodies, whose dimensions are small
compared with the distance AB, be charged with equal quantities
of positive electricity and placed at A and B respectively, and
let the charges be such that the force with which they repel each
other is the unit of force, measured as in Art. 6. Then the charge
of either body is said to be the unit of electricity. If the charge of
the body at B were a unit of negative electricity, then, since the
action between the bodies would be reversed, we should have an
attraction equal to the unit of force.
If the charge of A were also negative, and equal to unity, the
force would be repulsive, and equal to unity.
Since the action between any two portions of electricity is not
68.] LAW OF ELECTRIC FORCE. 69
affected by the presence of other portions, the repulsion between
e units of electricity at A and / units at B is <?/, the distance
AE being unity. See Art. 39.
Law of Force between Electrified Bodies.
66.] Coulomb shewed by experiment that the force between
electrified bodies whose dimensions are small compared with the
distance between them, varies inversely as the square of the dis
tance. Hence the actual repulsion between two such bodies charged
with quantities e and ef and placed at a distance r is
eef
f*'
We shall prove in Art. 74 that this law is the only one con
sistent with the observed fact that a conductor, placed in the inside
of a closed hollow conductor and in contact with it, is deprived of
all electrical charge. Our conviction of the accuracy of the law
of the inverse square of the distance may be considered to rest
on experiments of this kind, rather than on the direct measure
ments of Coulomb.
Resultant Force between Two Bodies.
67.] In order to find the resultant force between two bodies
we might divide each of them into its elements of volume, and
consider the repulsion between the electricity in each of the elements
of the first body and the electricity in each of the elements of the
second body. TVe should thus get a system of forces equal in
number to the product of the numbers of the elements into which
we have divided each body, and we should have to combine the
effects of these forces by the rules of Statics. Thus, to find the
component in the direction of x we should have to find the value
of the sextuple integral
rrrrrr P P' (*-*') dxdy a* d^d/M
JJJJJJ {(x-x'Y + (y-,,J + (z-zJ}?'
where #, y, z are the coordinates of a point in the first body at
which the electrical density is p, and x', y' ', /, and p are the
corresponding quantities for the second body, and the integration
is extended first over the one body and then over the other.
Resultant Force at a Point,
68.] In order to simplify the mathematical process, it is con
venient to consider the action of an electrified body, not on another
70 ELECTROSTATICS. [68.
body of any form, but on an indefinitely small body, charged with
an indefinitely small amount of electricity, and placed at any point
of the space to which the electrical action extends. By making
the charge of this body indefinitely small we render insensible its
disturbing action on the charge of the first body.
Let e be the charge of this body, and let the force acting on
it when placed at the point (#, y, z) be Re, and let the direction-
cosines of the force be I, m, n, then we may call R the resultant
force at the point (#, y, z).
In speaking of the resultant electrical force at a point, we do not
necessarily imply that any force is actually exerted there, but only
that if an electrified body were placed there it would be acted on
by a force Re, where e is the charge of the body.
Definition. The Resultant electrical force at any point is the
force which would be exerted on a small body charged with the unit
of positive electricity, if it were placed there without disturbing the
actual distribution of electricity.
This force not only tends to move an electrified body, but to
move the electricity within the body, so that the positive electricity
tends to move in the direction of R and the negative electricity
in the opposite direction. Hence the force R is also called the
Electromotive Force at the point (x, y, z).
When we wish to express the fact that the resultant force is a
vector, we shall denote it by the German letter @. If the body
is a dielectric, then, according to the theory adopted in this
treatise, the electricity is displaced within it, so that the quantity
of electricity which is forced in the direction of (£ across unit
of area fixed perpendicular to @ is
S) = ±-K<$'9
47T
where S) is the displacement, (§ the resultant force, and K the
specific inductive capacity of the dielectric. For air, K = 1.
If the body is a conductor, the state of constraint is continually
giving way, so that a current of conduction is produced and main
tained as long as the force @ acts on the medium.
Components of the Resultant Force.
If X, Y, Z denote the components of R, then
X=Rl, Y=Rm, Z=Rn-,
where /, m, n are the direction -cosines of R.
69.] ELECTROMOTIVE FORCE. 71
Line-Integral of Electric Force, or Electromotive Force along
an Arc of a Curve.
69.] The Electromotive force along a given arc AP of a curve is
numerically measured by the work which would be done on a unit
of positive electricity carried along the curve from the beginning,
A, to P, the end of the arc.
If s is the length of the arc, measured from A, and if the re
sultant force R at any point of the curve makes an angle c with
the tangent drawn in the positive direction, then the work done
on unit of electricity in moving along the element of the curve
ds will be
and the total electromotive force V will be
F= jfJiiooBc*,
"0
the integration being extended from the beginning to the end
of the arc.
If we make use of the components of the force R} we find
.
o ds ds ds'
If X, Y, and Z are such that Xda+Ydy + Zdz is a complete
differential of a function of x, y, z, then
where the integration is performed in any way from the point A
to the point P, whether along the given curve or along any other
line between A and P.
In this case V is a scalar function of the position of a point in
space, that is, when we know the coordinates of the point, the value
of V is determinate, and this value is independent of the position
and direction of the axes of reference. See Art. 16.
On Functions of the Position of a Point.
In what follows, when we describe a quantity as a function of
the position of a point, we mean that for every position of the point
the function has a determinate value. We do not imply that this
value can always be expressed by the same formula for all points of
space, for it may be expressed by one formula on one side of a
given surface and by another formula on the other side.
72 ELECTROSTATICS. [70.
On Potential Functions.
70.] The quantity Xdx+Ydy+Zdz is an exact differential
whenever the force arises from attractions or repulsions whose in
tensity is a function of the distance only from any number of
points. For if i\ be the distance of one of the points from the point
(#, y, z\ and if R^ be the repulsion, then
with similar expressions for Y1 and Z19 so that
Xl dx -\-Y1d^ + Z± dz = Rl dr^ ;
and since Rl is a function of rl only, Rl dr^ is an exact differential
of some function of rlt say V-^
Similarly for any other force R2, acting from a centre at dis
tance r2 ,
X2 dx + Y<idy + Z^ dz = Rz drz = dV^ .
But X = X1 -f X2 + &c. and Y and Z are compounded in the same
way, therefore
Xdx+Ydy + Zdz = d7i + dYt + &G. = dV.
V, the integral of this quantity, under the condition that V =. 0
at an infinite distance, is called the Potential Function.
The use of this function in the theory of attractions was intro
duced by Laplace in the calculation of the attraction of the earth.
Green, in his essay ' On the Application of Mathematical Analysis
to Electricity/ gave it the name of the Potential Function. Gauss,
working independently of Green, also used the word Potential.
Clausius and others have applied the term Potential to the work
which would be done if two bodies or systems were removed to
an infinite distance from one another. We shall follow the use of
the word in recent English works, and avoid ambiguity by adopting
the following definition due to Sir W. Thomson.
Definition of Potential. The Potential at a Point is the work
which would be done on a unit of positive electricity by the elec
tric forces if it were placed at that point without disturbing the
electric distribution, and carried from that point to an infinite
distance.
71.] Expressions for the Resultant Force and its components in
terms of the Potential.
Since the total electromotive force along any arc AB is
72.] POTENTIAL. 73
if we put ds for the arc AB we shall have for the force resolved
in the direction of ds,
7? dV
R cos e = -- 7-;
as
whence, by assuming ds parallel to each of the axes in succession,
we get dV dV dV
A = -- -j- i I = -- -=— j Z = -- — J
ax ay dz
• dy dz j '
We shall denote the force itself, whose magnitude is R and whose
components are X, Y, Z, by the German letter <£, as in Arts. 17
and 68.
The Potential at all Points within a Conductor is the same.
72.] A conductor is a body which allows the electricity within
it to move from one part of the body to any other when acted on
by electromotive force. When the electricity is in equilibrium
there can be no electromotive force acting within the conductor.
Hence R = 0 throughout the whole space occupied by the con
ductor. From this it follows that
dV _ dV _ dV _
~fa= °' ^ ' Tz~
and therefore for every point of the conductor
r= c,
where C is a constant quantity.
Potential of a Conductor.
Since the potential at all points within the substance of the
conductor is C, the quantity C is called the Potential of the con
ductor. C may be defined as the work which must be done by
external agency in order to bring a unit of electricity from an
infinite distance to the conductor, the distribution of electricity
being supposed not to be disturbed by the presence of the unit.
If two conductors have equal potentials, and are connected by
a wire so fine that the electricity on the wire itself may be neg
lected, the total electromotive force along the wire will be zero,
and no electricity will pass from the one conductor to the other.
If the potentials of the conductors A and B be VA and V^ then
the electromotive force along any wire joining A and B will be
rA-rB
74: ELECTROSTATICS. [73.
in the direction AB, that is, positive electricity will tend to pass
from the conductor of higher potential to the other.
Potential, in electrical science, has the same relation to Elec
tricity that Pressure, in Hydrostatics, has to Fluid, or that Tem
perature, in Thermodynamics, has to Heat. Electricity, Fluids,
and Heat all tend to pass from one place to another, if the Poten
tial, Pressure, or Temperature is greater in the first place than in
the second. A fluid is certainly a substance, heat is as certainly
not a substance, so that though we may find assistance from ana
logies of this kind in forming clear ideas of formal electrical rela
tions, we must be careful not to let the one or the other analogy
suggest to us that electricity is either a substance like water, or
a state of agitation like heat.
Potential due to any Electrical System.
73.] Let there be a single electrified point charged with a quantity
e of electricity, and let / be the distance of the point #',/, / from it,
then 7 = r Edr = f°4^ = -•
Jr Jr ?'2 r
Let there be any number of electrified points whose coordinates
are (^y^z^, (#2> ^2> ^2)' &c- and tneir charges elt e29 &c., and
let their distances from the point (#',./, /) be rl9 r2, &c., then the
potential of the system at x, y\ / will be
Let the electric density at any point (#, y, z) within an elec
trified body be p, then the potential due to the body is
where r= {(x-x'}* + (y-y')* +(z-z')2 }*,
the integration being extended throughout the body.
On the Proof of the Law of the Inverse Square.
74.] The fact that the force between electrified bodies is inversely
as the square of the distance may be considered to be established
by direct experiments with the torsion-balance. The results, how
ever, which we derive from such experiments must be regarded
as affected by an error depending on the probable error of each
experiment, and unless the skill of the operator be very great,
74.] PROOF OF THE LAW OF FORCE. 75
the probable error of an experiment with the torsion-balance is
considerable. As an argument that the attraction is really, and
not merely as a rough approximation, inversely as the square of the
distance, Experiment VII (p. 34) is far more conclusive than any
measurements of electrical forces can be.
In that experiment a conductor B, charged in any manner, was
enclosed in a hollow conducting vessel C, which completely sur
rounded it. C was also electrified in any manner.
B was then placed in electric communication with C, and was then
again insulated and removed from C without touching it, and ex
amined by means of an electroscope. In this way it was shewn
that a conductor, if made to touch the inside of a conducting vessel
which completely encloses it, becomes completely discharged, so
that no trace of electrification can be discovered by the most
delicate electrometer, however strongly the conductor or the vessel
has been previously electrified.
The methods of detecting the electrification of a body are so
delicate that a millionth part of the original electrification of B
could be observed if it existed. No experiments involving the direct
measurement of forces can be brought to such a degree of accuracy.
It follows from this experiment that a non-electrified body in the
inside of a hollow conductor is at the same potential as the hollow
conductor, in whatever way that conductor is charged. For if it
were not at the same potential, then, if it were put in electric
connexion with the vessel, either by touching it or by means of
a wire, electricity would pass from the one body to the other, and
the conductor, when removed from the vessel, would be found to be
electrified positively or negatively, which, as we have already stated,
is not the case.
Hence the whole space inside a hollow conductor is at the same
potential as the conductor if no electrified body is placed within it.
If the law of the inverse square is true, this will be the case what
ever be the form of the hollow conductor. Our object at present,
however, is to ascertain from this fact the form of the law of
attraction.
For this purpose let us suppose the hollow conductor to be a thin
spherical shell. Since everything is symmetrical about its centre,
the shell will be uniformly electrified at every point, and we have
to enquire what must be the law of attraction of a uniform spherical
shell, so as to fulfil the condition that the potential at every point
within it shall be the same.
76 ELECTROSTATICS. [74.
Let the force at a distance r from a point at which a quantity e
of electricity is concentrated be R, where R is some function of r.
All central forces which are functions of the distance admit of a
potential, let us write £i-£ for the potential function due to a unit
of electricity at a distance r.
Let the radius of the spherical shell be #, and let the surface-
density be a. Let P be any point within the shell at a distance
p from the centre. Take the radius through P as the axis of
spherical coordinates, and let r be the distance from P to an element
dS of the shell. Then the potential at P is
r=mm-*8,
' — I i
L L r
sn
o
Now r2 = a2 — 2 ajo cos 0 -f jo2,
r dr = ap sin 0 c?0.
Hence F= 2 TT <r -
xy /*a+p
- / /(r) dr ;
pJa-p
and F must be constant for all values of p less than a.
Multiplying both sides by p and differentiating with respect to p,
Differentiating again with respect to />,
0 =f(a+p)-f(a-p).
Since a and p are independent,
f (r) = C, a constant.
Hence f(r) = Cr+C',
and the potential function is
The force at distance r is got by differentiating this expression
with respect to r, and changing the sign, so that
/
or the force is inversely as the square of the distance, and this
therefore is the only law of force which satisfies the condition that
the potential within a uniform spherical shell is constant*. Now
* See Pratt' s Mechanical Philosophy, p. 144.
76.] ELECTRIC INDUCTION. 77
this condition is shewn to be fulfilled by the electric forces with
the most perfect accuracy. Hence the law of electric force is
verified to a corresponding degree of accuracy.
Surface-Integral of Electric Induction^ and Electric Displacement
through a Surface.
75.] Let R be the resultant force at any point of the surface,
and e the angle which R makes with the normal drawn towards the
positive side of the surface, then R cos e is the component of the
force normal to the surface, and if dS is the element of the surface,
the electric displacement through dS will be, by Art. 68,
— KR cos e dS.
4n
Since we do not at present consider any dielectric except air, K= 1 .
We may, however, avoid introducing at this stage the theory of
electric displacement, by calling R cos € dS the Induction through
the element dS. This quantity is well known in mathematical
physics, but the name of induction is borrowed from Faraday.
The surface-integral of induction is
JJ
R cos e dS,
and it appears by Art. 21, that if X, J", Z are the components of R,
and if these quantities are continuous within a region bounded by a
closed surface S, the induction reckoned from within outwards is
dX dY
Tx + ^ +
the integration being extended through the whole space within the
surface.
Induction through a Finite Closed Surface due to a Single Centre
of Force.
76.] Let a quantity e of electricity be supposed to be placed at a
point 0, and let r be the distance of any point P from 0, the force
Q
at that point is R = —in the direction OP.
Let a line be drawn from 0 in any direction to an infinite
distance. If 0 is without the closed surface this line will either
not cut the surface at all, or it will issue from the surface as many
times as it enters. If 0 is within the surface the line must first
78 ELECTROSTATICS. [76.
issue from the surface, and then it may enter and issue any number
of times alternately, ending- by issuing from it.
Let e be the angle between OP and the normal to the surface
drawn outwards where OP cuts it, then where the line issues from
the surface cos € will be positive, and where it enters cos e will
be negative.
Now let a sphere be described with centre 0 and radius unity,
and let the line OP describe a conical surface of small angular
aperture about 0 as vertex.
This cone will cut off a small element da from the surface of the
sphere, and small elements dSl} dS2, &c. from the closed surface at
the various places where the line OP intersects it.
Then, since any one of these elements dS intersects the cone at a
distance r from the vertex and at an obliquity €,
dS = r2 sec c da> ;
and, since R = er~2, we shall have
ficos€clS = ± edto ;
the positive sign being taken when r issues from the surface, and
the negative where it enters it.
If the point 0 is without the closed surface, the positive values
are equal in number to the negative ones, so that for any direction
of *> 2R cos e dS = 0,
and therefore / / R cos c dS = 0,
the integration being extended over the whole closed surface.
If the point 0 is within the closed surface the radius vector OP
first issues from the closed surface, giving a positive value of e da,
and then has an equal number of entrances and issues, so that in
this case SRcosedS = e d<*.
Extending the integration over the whole closed surface, we shall
include the whole of the spherical surface, the area of which is 47r,
so that
I I R cos e dS = e \ I da = 4 -n e.
Hence we conclude that the total induction outwards through a
closed surface due to a centre of force e placed at a point 0 is
zero when 0 is without the surface, and 47te when 0 is within
the surface.
Since in air the displacement is equal to the induction divided
77-] EQUATIONS OF LAPLACE AND POISSON. 79
by 4-7T, the displacement through a closed surface, reckoned out
wards, is equal to the electricity within the surface.
Corollary. It also follows that if the surface is not closed but
is bounded by a given closed curve, the total induction through
it is we, where o> is the solid angle subtended by the closed curve
at 0. This quantity, therefore, depends only on the closed curve,
and not on the form of the surface of which it is the boundary.
On the Equations of Laplace and Poisson.
77.] Since the value of the total induction of a single centre
of force through a closed surface depends only on whether the
centre is within the surface or not, and does not depend on its
position in any other way, if there are a number of such centres
el, £2, &c. within the surface, and £/, <?/, &c. without the surface,
we shall have
1 I RcosedS = btie;
where e denotes the algebraical sum of the quantities of elec
tricity at all the centres of force within the closed surface, that is,
the total electricity within the surface, resinous electricity being
reckoned negative.
If the electricity is so distributed within the surface that the
density is nowhere infinite, we shall have by Art. 64,
47T£ = 4-7T ]l I pdxdydz,
and by Art. 75,
t* AY dZ.
If we take as the closed surface that of the element of volume
dx dy dzy we shall have, by equating these expressions,
dX dY dZ
and if a potential V exists, we find by Art. 7 1 ,
This equation, in the case in which the density is zero, is called
Laplace's Equation. In its more general form it was first given by
Poisson. It enables us, when we know the potential at every point,
to determine the distribution of electricity.
80 ELECTROSTATICS. [78.
We shall denote, as at Art. 26, the quantity
and we may express Poisson's equation in words by saying that
the electric density multiplied by 4?r is the concentration of the
potential. Where there is no electrification, the potential . has no
concentration, and this is the interpretation of Laplace's equation.
If we suppose that in the superficial and linear distributions of
electricity the volume-density p remains finite, and that the elec
tricity exists in the form of a thin stratum or narrow fibre, then,
by increasing p and diminishing the depth of the stratum or the
section of the fibre, we may approach the limit of true superficial
or linear distribution, and the equation being true throughout the
process will remain true at the limit, if interpreted in accordance
with the actual circumstances.
On the Conditions to be fulfilled at an Electrified Surface.
78.] We shall consider the electrified surface as the limit to
which an electrified stratum of density p and thickness v approaches
when p is increased and v diminished without limit, the product pv
being always finite and equal to a- the surface-density.
Let the stratum be that included between the surfaces
F(vty,z) = F= a (1)
and F = a + h. (2)
If we put R2 =
dF
dx
dF
dy
dF
(3)
and if /, m, n are the direction-cosines of the normal to the surface,
jU dF dF dF m
Hi = -=- > Mm = -=— > lin = _ • MM
dx dy dz
Now let Fl be the value of the potential on the negative side
of the surface F = a, V its value between the surfaces F = a and
F = a + Tiy and V2 its value on the positive side of F — a + k.
Also, let pj, p', and p2 be the values of the density in these three
portions of space. Then, since the density is everywhere finite,
the second derivatives of V are everywhere finite, and the first
derivatives, and also the function itself, are everywhere continuous
and finite.
At any point of the surface F = a let a normal be drawn of
78.] ELECTRIFIED SURFACE. 81
length y, till it meets the surface F = a + h, then the value of F at
the extremity of the normal is
,dF dF dF.
or a + h = a + vR + &c. /// (6)
The value of V at the same point is
fldV dV dV\
^ = ^+*(l-^+m^ + ^) + &c, (7)
or r.-r^j^+ta. //y (8)
Since the first derivatives of V continue always finite, the second
side of the equation vanishes when Ji is diminished without limit,
and therefore if Y2 and V-± denote the values of V on the outside
and inside of an electrified surface at the point x, y, z,
7i = rt. 0)
If x -f dx, y + dy, z + dz be the coordinates of another point on
the electrified surface, F=a and 7^= T2 at this point also ; whence
dF, dF , dF
'
^-V^-f&c.; (11)
tt'A/
and when dx, dy, dz vanish, we find the conditions
— -7 — =• um, f (12)
dy dy
fjJL-.*%L=Ci
where C is a quantity to be determined.
dV
Next, let us consider the variation of F and -y- along the
ordinate parallel to x between the surfaces F= a and F = a + h.
dF cPF
We have F= a -f -j-dx + \ -r^(dx}2 + &c., (13)
dV d7l d^V Ld37' j 2 „
Hence, at the second surface, where F=a + k, and V becomes F2,
VOL. I. G
82 ELECTROSTATICS. [79.
whence -^- dx + &c. = Cl, (16)
by the first of equations (12).
Kf Multiplying by jfl, and remembering that at the second surface
Rldx=k (17)
we find -rTh=CJtl*. (18)
clx
Similarly &T ' _
(19)
and ~cT^^ = CRn*- (20)
Adding (-7-0- + -T-O- + -7-0-) A = CR ; (21)
\ //•7»^ r/'JJ CM 2 *
but -y-o- +.-T-O- + -TO- =—477^' and ^ = vR ; (22)
hence (7 = — 4 TT/V = — 4 w <r, (23)
where cr is the surface-density; or, multiplying the equations
(12) by I, m, n respectively, and adding,
This equation is called the characteristic equation of V at a surface.
This equation may also be written
dV, dF2
where rl5 z^2 are the normals to the surface drawn towards the
first and the second medium respectively, and 7^, T2 the potentials
at points on these normals. We may also write it
-S2coS€2 + JS1cose1 + 47ro- = 0 ; (26)
where R^ Rz are the resultant forces, and clt e2 the angles which
they make with the normals drawn from the surface on either
side.
79.] Let us next determine the total mechanical force acting on
an element of the electrified surface.
The general expression for the force parallel to x on an element
whose volume is dx dy dz, and volume-density p, is
dX = — -=- p dx dy dz. (27)
80.] FORCE ACTING ON AN ELECTRIFIED SURFACE. 83
In the present case we have for any point on the normal v
dV dV, d*~PS
(28)
' -
dx dx dxfyiv
also, if the element of surface is dS, that of the volume of the
element of the stratum may be written dSdv ; and if X is the whole
force on a stratum of thickness v,
. (29)
Integrating with respect to v, we find
" (30)
evnce . = ++c.; (31)
(32)
When v is diminished and // increased without limit, the product
p'v remaining always constant and equal to o-, the expression for
the force in the direction of x on the electricity a- dS on the element
of surface (IS is ^ 701X^1 dT9\
X=-^4(-^ + -^); (33)
that is, the force acting on the electrified element o- dS in any given
direction is the arithmetic mean of the forces acting on equal
quantities of electricity placed one just inside the surface and the
other just outside the surface close to the actual position of the
element, and therefore the resultant mechanical force on the elec
trified element is equal to the resultant of the forces which would
act on two portions of electricity, each equal to half that on the
element, and placed one on each side of the surface and infinitely
near to it.
80.] When a conductor is in electrical equilibrium, the whole of the
electricity is on the surface.
We have already shewn that throughout the substance of the
conductor the potential V is constant. Hence y2 V is zero, and
therefore by Poisson's equation, p is zero throughout the substance
of the conductor, and there can be no electricity in the interior
of the conductor.
Hence a superficial distribution of electricity is the only possible
one in the case of conductors in equilibrium. A distribution
throughout the mass can only exist in equilibrium when the body
is a non-conductor.
G 2
84 ELECTKOSTATICS. [8 1.
Since the resultant force within a conductor is zero, the resultant
force just outside the conductor is along the normal and is equal to
4 TT a; acting outwards from the conductor.
81.] If we now suppose an elongated body to be electrified, we
may, by diminishing its lateral dimensions, arrive at the conception
of an electrified line.
Let ds be the length of a small portion of the elongated body,
and let c be its circumference, and o- the superficial density of the
electricity on its surface ; then, if A. is the electricity per unit of
length, A. = co-, and the resultant electrical force close to the
surface will be A
477 (T = 4 77 — •
C
If, while X remains finite, c be diminished indefinitely, the force
at the surface will be increased indefinitely. Now in every di
electric there is a limit beyond which the force cannot be increased
without a disruptive discharge. Hence a distribution of electricity
in which a finite quantity is placed on a finite portion of a line
is inconsistent with the conditions existing in nature.
Even if an insulator could be found such that no discharge could
be driven through it by an infinite force, it would be impossible
to charge a linear conductor with a finite quantity of electricity,
for an infinite electromotive force would be required to bring the
electricity to the linear conductor.
In the same way it may be shewn that a point charged with
a finite quantity of electricity cannot exist in nature. It is con
venient, however, in certain cases, to speak of electrified lines and
points, and we may suppose these represented by electrified wires,
and by small bodies of which the dimensions are negligible com
pared with the principal distances concerned.
Since the quantity of electricity on any given portion of a wire
diminishes indefinitely when the diameter of the wire is indefinitely
diminished, the distribution of electricity on bodies of considerable
dimensions will not be sensibly affected by the introduction of very
fine metallic wires into the field, so as to form electrical connexions
between these bodies and the earth, an electrical machine, or an
electrometer.
On Lines of Force.
82.] If a line be drawn whose direction at every point of its
course coincides with that of the resultant force at that point, the
line is called a Line of Force.
82.] LINES OF FOKCE. 85
If lines of force be drawn from every point of a line they will
form a surface such that the force at any point is parallel to the
tangent plane at that point. The surface-integral of the force with
respect to this surface or any part of it will therefore be zero.
If lines of force are drawn from every point of a closed curve Ll
they will form a tubular surface S0. Let the surface S19 bounded
by the closed curve Llt be a section of this tube, and let S2 be any
other section of the tube. Let Q0, Q19 Q2 be the surface-integrals
over S0) Slt S2, then, since the three surfaces completely enclose a
space in which there is no attracting matter, we have
Qo+Qi+ Qz = 0.
But §o = °> therefore Q2= — Qlt or the surface-integral over
the second section is equal and opposite to that over the first : but
since the directions of the normal are opposite in the two cases, we
may say that the surface-integrals of the two sections are equal, the
direction of the line of force being supposed positive in both.
Such a tube is called a Solenoid*, and such a distribution of
force is called a Solenoidal distribution. The velocities of an in
compressible fluid are distributed in this manner.
If we suppose any surface divided into elementary portions such
that the surface-integral of each element is unity, and if solenoids
are drawn through the field of force having these elements for their
bases, then the surface-integral for any other surface will be re
presented by the number of solenoids which it cuts. It is in this
sense that Faraday uses his conception of lines of force to indicate
not only the direction but the amount of the force at any place in
the field.
We have used the phrase Lines of Force because it has been used
by Faraday and others. In strictness, however, these lines should
be called Lines of Electric Induction.
In the ordinary cases the lines of induction indicate the direction
and magnitude of the resultant electromotive force at every point,
because the force and the induction are in the same direction and
in a constant ratio. There are other cases, however, in which it
is important to remember that these lines indicate the induction,
and that the force is indicated by the equipotential surfaces, being
normal to these surfaces and inversely proportional to the distances
of consecutive surfaces.
* From ffw\r)i>, a tube. Faraday uses (3271) the term 'Sphondyloid' in the same
sense.
86 ELECTROSTATICS. [83.
On Specific Inductive Capacity.
83.] In the preceding investigation of surface-integrals I have
adopted the ordinary conception of direct action at a distance,
and have not taken into consideration any effects depending on the
nature of the dielectric medium in which the forces are observed.
But Faraday has observed that the quantity of electricity
induced by a given electromotive force on the surface of a conductor
which bounds a dielectric is not the same for all dielectrics. The
induced electricity is greater for most solid and liquid dielectrics
than for air and gases. Hence these bodies are said to have a
greater specific inductive capacity than air, which is the standard
medium.
We may express the theory of Faraday in mathematical language
by saying that in a dielectric medium the induction across any
surface is the product of the normal electric force into the coefficient
of specific inductive capacity of that medium. If we denote this
coefficient by K, then in every part of the investigation of sur
face-integrals we must multiply X, I7", and Z by K, so that the
equation of Poisson will become
d ^dV d vdV d ^dV
-=-.K^- + -j-.K-j- + -j-.K^-+4;Trp = 0.
dx dx dy dy dz dz
At the surface of separation of two media whose inductive capa
cities are K± and K2, and in which the potentials are V± and T2i
the characteristic equation may be written
where v is the normal drawn from the first medium to the second,
and o- is the true surface-density on the surface of separation ;
that is to say, the quantity of electricity which is actually on the
surface in the form of a charge, and which can be altered only by
conveying electricity to or from the spot. This true electrification
must be distinguished from the apparent electrification (/_, which is
the electrification as deduced from the electrical forces in the neigh
bourhood of the surface, using the ordinary characteristic equation
If a solid dielectric of any form is a perfect insulator, and if
its surface receives no charge, then the true electrification remains
zero, whatever be the electrical forces acting on it.
83.] SPECIFIC INDUCTIVE CAPACITY. 87
dF2 K^dVi ,
Hence T, = T2 -d^> and
dV 4V(/K
The surface-density </ is that of the apparent electrification
produced at the surface of the solid dielectric by induction. It
disappears entirely when the inducing force is removed, but if
during the action of the inducing force the apparent electrification
of the surface is discharged by passing a flame over the surface,
then, when the inducing force is taken away, there will appear an
electrification opposite to <r' *.
In a heterogeneous dielectric in which K varies continuously, if
p be the apparent volume-density,
Comparing this with the equation above, we find
dKdV dKdV dKdV
The true electrification, indicated by p, in the dielectric whose
variable inductive capacity is denoted by K, will produce the same
potential at every point as the apparent electrification, indicated by
p7, would produce in a dielectric whose inductive capacity is every
where equal to unity.
* See Faraday's 'Kemarks on Static Induction,' Proceedings of the Royal In
stitution, Feb. 12, 1858.
CHAPTER III.
SYSTEMS OP CONDUCTORS.
On the Superposition of Electrical Systems.
84.] Let El be a given electrified system of which the potential
at a point P is T1} and let U2 be another electrified system of which
the potential at the same point would be F2 if El did not exist.
Then, if E± and Ez exist together, the potential of the combined
system will be /^-f F2.
Hence, if V be the potential of an electrified system E, if the
electrification of every part of E be increased in the ratio of n to 1 ,
the potential of the new system nE will be n V.
Energy of an Electrified System.
85.] Let the system be divided into parts, A19 A2, &c. so small
that the potential in each part may be considered constant through
out its extent. Let el, £2, &c. be the quantities of electricity in
each of these parts, and let T19 F"2, &c. be their potentials.
If now e1 is altered to ne^ e% to ne^ &c., then the potentials will
become nTlt nV^ &c.
Let us consider the effect of changing n into n + dn in all these
expressions. It will be equivalent to charging A1 with a quantity
of electricity el dn, A2 with e2dn, &c. These charges must be sup
posed to be brought from a distance at which the electrical action
of the system is insensible. The work done in bringing e1 dn of
electricity to A19 whose potential before the charge is nV19 and after
the charge (n + dn) Flf must lie between
n Fj e1 dn and (n -f dn) V^ e± dn.
In the limit we may neglect the square of dn, and write the
expression
86.] COEFFICIENTS OF POTENTIAL AND OF INDUCTION. 89
Similarly the work required to increase the charge of A^ is
~P2e2ndn, so that the whole work done in increasing the charge
of the system is
If we suppose this process repeated an indefinitely great number
of times, each charge being indefinitely small, till the total effect
becomes sensible, the work done will be
2 ( 7e)fn tin = J 2 ( Ve] (n*-n*) ;
where 2 ( Ve) means the sum of all the products of the potential of
each element into the quantity of electricity in that element when
n — 1, and nQ is the initial and % the final value of n.
If we make n0 = 0 and % = I, we find for the work required to
charge an unelectrified system so that the electricity is e and the
potential V in each element,
General Theory of a System of Conductors.
86.] Let A i, A2, ...An be any number of conductors of any
form. Let the charge or total quantity of electricity on each of
these be E^ E.2J ... En3 and let their potentials be T19 F2, ... Jn
respectively.
Let us suppose the conductors to be all insulated and originally
free of charge, and at potential zero.
Now let A1 be charged with unit of electricity, the other bodies
being without charge. The effect of this charge on A1 will be to
raise the potential of Al tojon, that of A2 to p^, and that of An to
j}ln, where j»u, &c. are quantities depending on the form and rela
tive position of the conductors. The quantity j»n may be called the
Potential Coefficient of Al on itself, and pl2 may be called the Po
tential Coefficient of A± on A2, and so on.
If the charge upon A± is now made El , then, by the principle of
superposition, we shall have
Now let A1 be discharged, and A2 charged with unit of electricity,
and let the potentials of Alt A2, ... An be ^21^22? --Pzn
potentials due to E.2 on A^ will be
Similarly let us denote the potential of As due to a unit charge
on Ar by j)rs, and let us call^r5 the Potential Coefficient of Ar on As,
90 SYSTEMS OF CONDUCTORS. [87.
then we shall have the following equations determining the po
tentials in terms of the charges :
(1)
We have here n linear equations containing n2 coefficients of
potential.
87.] By solving these equations for E19 E2, &c. we should obtain
n equations of the form
(2)
n n • . • .. • • nnn'
The coefficients in these equations may be obtained directly from
those in the former equations. They may be called Coefficients of
Induction.
Of these qn is numerically equal to the quantity of electricity
on Al when Al is at potential unity and all the other bodies are
at potential zero. This is called the Capacity of A^ It depends
on the form and position of all the conductors in the system.
Of the rest qrs is the charge induced on Ar when As is main
tained at potential unity and all the other conductors at potential
zero. This is called the Coefficient of Induction of A8 on Ar.
The mathematical determination of the coefficients of potential
and of capacity from the known forms and positions of the con
ductors is in general difficult. We shall afterwards prove that they
have always determinate values, and we shall determine their values
in certain special cases. For the present, however, we may suppose
them to be determined by actual experiment.
Dimensions of these Coefficients.
Since the potential of an electrified point at a distance r is the
charge of electricity divided by the distance, the ratio of a quantity
of electricity to a potential may be represented by a line. Hence
all the coefficients of capacity and induction (q) are of the nature of
lines, and the coefficients of potential (p) are of the nature of the
reciprocals of lines.
88.] RECIPROCAL PROPERTY OF THE COEFFICIENTS. 91
88.] THEOREM I. The coefficients of Ar relative to A8 are equal to
those of A8 relative to Ar.
If Er, the charge on Ar, is increased by bflr, the work spent in
bringing bflr from an infinite distance to the conductor Ar whose
potential is Vry is by the definition of potential in Art. 70,
rr*Er,
and this expresses the increment of the electric energy caused by
this increment of charge.
If the charges of the different conductors are increased by
&c., the increment of the electric energy of the system will be
If, therefore, the electric energy Q is expressed as a function
of the charges £lt E.2, &c., the potential of any conductor may be
expressed as the partial differential coefficient of this function with
respect to the charge on that conductor, or
Since the potentials are linear functions of the charges, the energy
must be a quadratic function of the charges. If we put
CErEs
for the term in the expansion of Q which involves the product
ErE8, then, by differentiating with respect to Es, we find the term
of the expansion of Vs which involves Er to be CEr .
Differentiating with respect to Er, we find the term in the
expansion of Vr which involves Es to be CES.
Comparing these results with equations (1), Art. 86, we find
Prs = C = Psr,
or, interpreting the symbols prs and psr : —
The potential of A8 due to a unit charge on Ar is equal to the
potential of Ar due to a unit charge on As .
This reciprocal property of the electrical action of one conductor
on another was established by Helmholtz and Sir W. Thomson.
If we suppose the conductors Ar and As to be indefinitely small,
we have the following reciprocal property of any two points : —
The potential at any point A8 , due to unit of electricity placed
at Ar in presence of any system of conductors, is a function of the
positions of Ar and A8 in which the coordinates of Ar and of As
enter in the same manner, so that the value of the function is
unchanged if we exchange Ar and Af .
92 SYSTEMS OF CONDUCTOKS. [89.
This function is known by the name of Green's Function.
The coefficients of induction qrs and qsr are also equal. This is
easily seen from the process by which these coefficients are obtained
from the coefficients of potential. For, in the expression for qrs,
prs and psr enter in the same way as psr and prs do in the expression
for qsr . Hence if all pairs of coefficients prs and psr are equal, the
pairs qrs and qsr are also equal.
»
89.] THEOREM II. Let a charge Er be placed on Ar) and let all
the other conductors he at potential zero, and let the charge
induced on A8 be — nrsEr, then if Ar is discharged and insulated,
and As brought to potential V8, the other conductors being at
potential zero} then the potential of Ar will be + nrs~P~8.
For, in the first case, if Vr is the potential of Ar, we find by
equations (2),
E8 = qrsYr, and Er = qrr7r.
Hence E8 = ^Er, and nrs = - ^ •
qrr qrr
In the second case, we have
Hence Vr = -V8 = nrj%.
"rr
From this follows the important theorem, due to Green : —
If a charge unity, placed on the conductor AQ in presence of
conductors A19 A2, &c. at potential zero induces charges — nlt
— n2, &c. in these conductors, then, if AQ is discharged and in
sulated, and these conductors are maintained at potentials V^ T2,
&c., the potential of A0 will be
The quantities (n) are evidently numerical quantities, or ratios.
The conductor A0 may be supposed reduced to a point, and
A19 A2, &c. need not be insulated from each other, but may be
different elementary portions of the surface of the same conductor.
We shall see the application of this principle when we investigate
Green's Functions.
90.] THEOREM III. The coefficients of potential are all positive,
but none of the coefficients pr8 is greater thanprr or p8S.
For let a charge unity be communicated to Ar, the other con
ductors being uncharged. A system of equipotential surfaces will
91.] PROPERTIES OF THE COEFFICIENTS. 93
be formed. Of these one will be the surface of Ari and its potential
will be prr . If Af is placed in a hollow excavated in Ar so as to be
completely enclosed by it, then the potential of As will also be p^.
If, however, Ag is outside of Ar its potential prs will lie between
prr and zero.
For consider the lines of force issuing from the charged con
ductor Ar. The charge is measured by the excess of the number
of lines which issue from it over those which terminate in it.
Hence, if the conductor has no charge, the number of lines which
enter the conductor must be equal to the number which issue from
it. The lines which enter the conductor come from places of greater
potential, and those which issue from it go to places of less poten
tial. Hence the potential of an uncharged conductor must be
intermediate between the highest and lowest potentials in the field,
and therefore the highest and lowest potentials cannot belong to
any of the uncharged bodies.
The highest potential must therefore be prr, that of the charged
body Ar, and the lowest must be that of space at an infinite dis
tance, which is zero, and all the other potentials such as pra must
lie between prr and zero.
If A8 completely surrounds A^ then^rs = pri.
91.] THEOREM IV. None of the coefficients of induction are positive,
and the sum of all those belonging to a single conductor is not
numerically greater than the coefficient of capacity of that con
ductor, which is always positive.
For let Ar be maintained at potential unity while all the other
conductors are kept at potential zero, then the charge on Ar is q^,
and that on any other conductor As is qrs .
The number of lines of force which issue from Ar isprr. Of these
some terminate in the other conductors, and some may proceed to
infinity, but no lines of force can pass between any of the other
conductors or from them to infinity, because they are all at potential
zero.
No line of force can issue from any of the other conductors such
as As, because no part of the field has a lower potential than As.
If As is completely cut off from Ar by the closed surface of one
of the conductors, then qrs is zero. If As is not thus cut off, qrs is a
negative quantity.
If one of the conductors At completely surrounds Ar, then all
the lines of force from Ar fall on At and the conductors within it,
94 SYSTEMS OF CONDUCTORS. [92.
and the sum of the coefficients of induction of these conductors with
respect to Ar will be equal to qrr with its sign changed. But if
Ar is not completely surrounded by a conductor the arithmetical
sum of the coefficients of induction qrs, &c. will be less than qrr.
We have deduced these two theorems independently by means
of electrical considerations. We may leave it to the mathematical
student to determine whether one is a mathematical consequence
of the other.
Resultant Mechanical Force on any Conductor in terms of the Charges.
92.] Let 8$ be any mechanical displacement of the conductor,
and let 4> be the the component of the force tending to produce that
displacement, then <J>8<£ is the work done by the force during
the displacement. If this work is derived from the electrification
of the system, then if Q is the electric energy of the system,
= 0, (3)
Here Q = i (El 71 + E2 F2 + &c.) (5)
If the bodies are insulated, the variation of Q must be such that
E^ EZJ &c. remain constant. Substituting therefore for the values
of the potentials, we have
Q = 4Sr S.(*, •&.*„), (6)
where the symbol of summation 2 includes all terms of the form
within the brackets, and r and s may each have any values from
1 to n. From this we find
as the expression for the component of the force which produces
variation of the generalized coordinate </>.
Resultant Mechanical Force in terms of the Potentials.
93.] The expression for <I> in terms of the charges is
*=-iSrS.(*rfl.f!p, /'I../;.. (8)
where in the summation r and «s have each every value in suc
cession from 1 to n.
Now Er = 2j ( %qrt) where t may have any value from 1 to n,
so that
93-] RESULTANT FORCE IN TERMS OF POTENTIALS. 95
*=-kWSt(Wan%f). - (9)
Now the coefficients of potential are connected with those of
induction by n equations of the form
Sr(Arfcr)= 1, (10)
and \n(n—\) of the form
SrQ»«rfrr) = 0. (11)
Differentiating with respect to <£ we get %n(n + 1) equations of
the form ^ ^
M*-|?)+Mfc^)=0, (12)
where a and 3 may be the same or different.
Hence, putting a and b equal to r and s,
(13)
but 2g (figure) =• Vr, so that we may write
* = 4S,S,(J^fe), (14)
where r and £ may have each every value in succession from 1
to n. This expression gives the resultant force in terms of the
potentials.
If each conductor is connected with a battery or other con
trivance by which its potential is maintained constant during the
displacement, then this expression is simply
under the condition that all the potentials are constant.
The work done in this case during the displacement 8<£ is 4>6$,
and the electrical energy of the system of conductors is increased
by 8Q; hence the energy spent by the batteries during the dis
placement is
(16)
It appears from Art. 92, that the resultant force <£ is equal to
— ~ , under the condition that the charges of the conductors are
* dQ
constant. It is also, by Art. 93, equal to y^, under the con
dition that the potentials of the conductors are constant. If the
conductors are insulated, they tend to move so that their energy
is diminished, and the work done by the electrical forces during
the displacement is equal to the diminution of energy.
If the conductors are connected with batteries, so that their
96 SYSTEMS OF CONDUCTORS. [94.
potentials are maintained constant, they tend to move so that the
energy of the system is increased, and the work done by the
electrical forces during the displacement is equal to the increment
of the energy of the system. The energy spent by the batteries
is equal to double of either of these quantities, and is spent half
in mechanical, and half in electrical work.
On the Comparison of Similar Electrified Systems.
94.] If two electrified systems are similar in a geometrical sense.,
so that the lengths of corresponding lines in the two systems
are as L to L', then if the dielectric which separates the conducting
bodies is the same in both systems, the coefficients of induction
and of capacity will be in the proportion of L to U '. For if we
consider corresponding portions, A and A', of the two systems, and
suppose the quantity of electricity on A to be E, and that on A'
to be E', then the potentials V and V at corresponding points
B and ^, due to this electrification, will be
E
But AS is to A'l? as L to L', so that we must have
E:E' :'.L7:L'7'.
But if the inductive capacity of the dielectric is different in the
two systems, being K in the first and K' in the second, then if the
potential at any point of the first system is to that at the cor
responding point of the second as V to V'9 and if the quantities
of electricity on corresponding parts are as E to E't we shall have
By this proportion we may find the relation between the total
electrification of corresponding parts of two systems, which are
in the first place geometrically similar, in the second place com
posed of dielectric media of which the dielectric inductive capacity
at corresponding points is in the proportion of K to K'9 and in
the third place so electrified that the potentials of corresponding
points are as V to V.
From this it appears that if q be any coefficient of capacity or
induction in the first system, and c[ the corresponding one in the
second> q : q' : : LK : L'K' ;
and if p and p' denote corresponding coefficients of potential in
the two systems, 1 1
94-] COMPARISON OF SIMILAR SYSTEMS. 97
If one of the bodies be displaced in the first system, and the
corresponding body in the second system receive a similar dis
placement, then these displacements are in the proportion of L
to If, and if the forces acting on the two bodies are as F to F,
then the work done in the two systems will be as FL to F'L '.
But the total electrical energy is half the sum of the quantities
of electricity multiplied each by the potential of the electrified
body, so that in the similar systems, if Q and Q' be the total
electrical energy,
Q : qf : : E7 : E' V ,
and the difference of energy after similar displacements in the two
systems will be in the same proportion. Hence, since FL is pro
portional to the electrical work done during the displacement,
FL :F'L' ::E7:E'Y'.
Combining these proportions, we find that the ratio of the
resultant force on any body of the first system to that on the
corresponding body of the second system is
F : F' : :
or
' L'2K'
The first of these proportions shews that in similar systems the
force is proportional to the square of the electromotive force and
to the inductive capacity of the dielectric, but is independent of the
actual dimensions of the system.
Hence two conductors placed in a liquid whose inductive capacity
is greater than that of air, and electrified to given potentials, will
attract each other more than if they had been electrified to the
same potentials in air.
The second proportion shews that if the quantity of electricity
on each body is given, the forces are proportional to the squares
of the electrifications and inversely to the squares of the distances,
and also inversely to the inductive capacities of the media.
Hence, if two conductors with given charges are placed in a
liquid whose inductive capacity is greater than that of air, they
will attract each other less than if they had been surrounded with
air and electrified with the same charges of electricity.
VOL. I.
CHAPTER IV.
GENERAL THEOREMS.
95.] IN the preceding chapter we have calculated the potential
function and investigated its properties on the hypothesis that
there is a direct action at a distance between electrified bodies,
which is the resultant of the direct actions between the various
electrified parts of the bodies.
If we call this the direct method of investigation, the inverse
method will consist in assuming that the potential is a function
characterised by properties the same as those which we have already
established, and investigating the form of the function.
In the direct method the potential is calculated from the dis
tribution of electricity by a process of integration, and is found
to satisfy certain partial differential equations. In the inverse
method the partial differential equations are supposed given, and
we have to find the potential and the distribution of electricity.
It is only in problems in which the distribution of electricity
is given that the direct method can be used. When we have to
find the distribution on a conductor we must make use of the
inverse method.
"We have now to shew that the inverse method leads in every
case to a determinate result, and to establish certain general
theorems deduced from Poisson's partial differential equation
The mathematical ideas expressed by this equation are of a
different kind from those expressed by the equation
/+GO r + ao r + x> «
/ / t
•00 J — CD J — 00 /
dsf.
In the differential equation we express that the values of the
second derivatives of V in the neighbourhood of any point, and
96.] CHARACTERISTICS OF THE POTENTIAL. 99
the density at that point are related to each other in a certain
manner, and no relation is expressed between the value of V at
that point and the value of p at any point at a sensible distance
from it.
In the second expression, on the other hand, the distance between
the point (x 3 y', z') at which p exists from the point (#, y, z] at
which V exists is denoted by r, and is distinctly recognised in the
expression to be integrated.
The integral, therefore, is the appropriate mathematical expression
for a theory of action between particles at a distance, whereas the
differential equation is the appropriate expression for a theory of
action exerted between contiguous parts of a medium.
We have seen that the result of the integration satisfies the
differential equation. We have now to shew that it is the only
solution of that equation fulfilling certain conditions.
We shall in this way not only establish the mathematical equi
valence of the two expressions, but prepare our minds to pass from
the theory of direct action at a distance to that of action between
contiguous parts of a medium.
Characteristics of the Potential Function.
96.] The potential function V, considered as derived by integration
from a known distribution of electricity either in the substance of
bodies with the volume-density p or on certain surfaces with the
surface-density a, p and a- being everywhere finite, has been shewn
to have the following characteristics : —
(1) Fis finite and continuous throughout all space.
(2) V vanishes at an infinite distance from the electrified system.
(3) The first derivatives of V are finite throughout all space, and
continuous except at the electrified surfaces.
(4) At every point of space, except on the electrified surfaces, the
equation of Poisson
is satisfied. We shall refer to this equation as the General
Characteristic equation.
At every point where there is no electrification this equation
becomes the equation of Laplace,
100 GENERAL THEOREMS. [97.
(5) At any point of an electrified surface at which the surface-
density is cr, the first derivative of F, taken with respect to the
normal to the surface, changes its value abruptly at the surface,
so that Ayf dV
where v and // are the normals on either side of the surface, and
V and V are the corresponding- potentials. We shall refer to this
equation as the Superficial Characteristic equation.
(G) If V denote the potential at a point whose distance from
any fixed point in a finite electrical system is r, then the product
Vr, when r increases indefinitely, is ultimately equal to E, the total
charge in the finite system.
97.] Lemma. Let V be any continuous function of x, yy z, and
let u, v, w be functions of #, y, z, subject to the general solenoidal
condition du dv dw
Tx + Ty + -dz = *>
where these functions are continuous, and to the superficial sole
noidal condition
l(ul — u2} + m(v^-v2]-\-n(wl — w^ = 0, (2)
at any surface at which these functions become discontinuous,
£, m, n being the direction-cosines of the normal to the surface,
and ultvlt wl and u2, v2, w2 the values of the functions on opposite
sides of the surface, then the triple integral
(fff dV dV dV^ .
M = I I I (u -j- + v -j- + w -=- ) dx dy dz (3)
J J J ^ dx dy dz'
vanishes when the integration is extended over a space bounded by
surfaces at which either V is constant, or
lu + mv + nw = 0, (4)
/, m, n, being the direction-cosines of the surface.
Before proceeding to prove this theorem analytically we may
observe, that if u, v, w be taken to represent the components of the
velocity of a homogeneous incompressible fluid of density unity,
and if V be taken to represent the potential at any point of space
of forces acting on the fluid, then the general and superficial equa
tions of continuity ((1) and (2)) indicate that every part of the
space is, and continues to be, full of the fluid, and equation (4)
is the condition to be fulfilled at a surface through which the fluid
does not pass.
The integral M represents the work done by the fluid against
the forces acting on it in unit of time.
97-] LEMMA.
Now, since the forces which act on the fluid are derived from
the potential function F} the work which they do is subject to the
law of conservation of energy, and the work done on the whole
fluid within a certain space may be found if we know the potential
at the points where each line of flow enters the space and where
it issues from it. The excess of the second of these potentials over
the first, multiplied by the quantity of fluid which is transmitted
along each line of flow, will give the work done by that portion
of the fluid, and the sum of all such products will give the whole
work.
Now, if the space be bounded by a surface for which V= C} a
constant quantity, the potential will be the same at the place
where any line of flow enters the space and where it issues from
it, so that in this case no work will be done by the forces on the
fluid within the space, and M = 0.
Secondly, if the space be bounded in whole or in part by a
surface satisfying equation (4), no fluid will enter or leave the space
through this surface, so that no part of the value of M can depend
on this part of the surface.
The quantity M is therefore zero for a space bounded externally
by the closed surface F= C, and it remains zero though any part
of this space be cut off from the rest by surfaces fulfilling the
condition (4).
The analytical expression of the process by which we deduce the
work done in the interior of the space from that which takes place
at the bounding surface is contained in the following method of
integration by parts.
Taking the first term of the integral M,
where 2 (u F) = u± 7l —n2F2 + UB F3 — ?/4 F4 + &c. ;
and where %F15 w2F2, &c. are the values of u and v at the points
whose coordinates are (alt y, z), (x.2) y, z), &c., sfly a?2, &c. being the
values of x where the ordinate cuts the bounding surface or surfaces,
arranged in descending order of magnitude.
Adding the two other terms of the integral M, we find
J/ =
102 GENERAL THEOREMS. [97.
If l} m, n are the direction-cosines of the normal drawn inwards
from the bounding surface at any point, and dS an element of that
surface, then we may write
the integration of the first term being extended over the bounding
surface, and that of the second throughout the entire space.
For all spaces within which u, v, w are continuous, the second
term vanishes in virtue of equation (1). If for any surface within
the space u, v, w are discontinuous but subject to equation (2), we
find for the part of M depending on this surface,
= — / /
= — / /
t) dS
I)
2'
where the suffixes ^ and 2, applied to any symbol, indicate to which
of the two spaces separated by the surface the symbol belongs.
Now, since V is continuous, we have at every point of the surface,
F1=F2=F;
we have also dSl = dS2 = d8;
but since the normals are drawn in opposite directions, we have
/! = — 12 = I, m1 = — m2 = m, % = — n2 — n ;
so that the total value of M, so far as it depends on the surface of
discontinuity, is
The quantity under the integral sign vanishes at every point in
virtue of the superficial solenoidal condition or characteristic (2).
Hence, in determining the value of M, we have only to consider
the surface-integral over the actual bounding surface of the space
considered, or
M = — F(lu + mv + nw)dS.
Case 1 . If V is constant over the whole surface and equal to (7,
(lu + mv + nw] dS.
= — C(
The part of this expression under the sign of double integration
represents the surface-integral of the flux whose components are
u, v, w, and by Art. 2 1 this surface-integral is zero for the closed
surface in virtue of the general and superficial solenoidal conditions
(1) and (2).
98.] THOMSON'S THEOREM. 103
Hence M = 0 for a space bounded by a single equipotential
surface.
If the space is bounded externally by the surface V •=. C, and
internally by the surfaces 7 ' — Cl} F= C2, &c., then the total value
of M for the space so bounded will be
Jf-JKi-j^&c.,
where M is the value of the integral for the whole space within the
surface V = C, and Ml , M2 are the values of the integral for the
spaces within the internal surfaces. But we have seen that M}
M!, M2, &c. are each of them zero, so that the integral is zero also
for the periphractic region between the surfaces.
Case 2. If lu + mv + nw is zero over any part of the bounding
surface, that part of the surface can contribute nothing to the value
of 31, because the quantity under the integral sign is everywhere
zero. Hence M will remain zero if a surface fulfilling this con
dition is substituted for any part of the bounding surface, provided
that the remainder of the surface is all at the same potential.
98.] We are now prepared to prove a theorem which we owe to
Sir William Thomson *.
As we shall require this theorem in various parts of our subject,
I shall put it in a form capable of the necessary modifications.
Let a, 6, c be any functions of x, y, z (we may call them the
components of a flux) subject only to the condition
da db dc
-J- + -j- + ~T
dx dy dz
where p has given values within a certain space. This is the general
characteristic of a, b, c.
Let us also suppose that at certain surfaces (S) a, b, and c are
discontinuous, but satisfy the condition
I(al—aj + m(61—6j + n(c1 — cj + lv<r = 0; (6)
where I, m, n are the direction-cosines of the normal to the surface,
aL, 11} q the values of a, b, c on the positive side of the surface, and
a2, b2, c.2 those on the negative side, and o- a quantity given for
every point of the surface. This condition is the superficial charac
teristic of a, b, c.
Next, let us suppose that V is a continuous function of #, y, z,
which either vanishes at infinity or whose value at a certain point
is given, and let V satisfy the general characteristic equation
* Cambridge and Dublin Mathematical Journal, February, 1848.
104 GEKEKAL THEOREMS. [98
d dV d d7 d dV
0', (7)
dx dy dy dz dz
«/ «/
and the superficial characteristic at the surfaces
7/rr dV* „ dV<>\ /xr dF-, „ dV^
I (JT, -j-i — JC *\ + m(K 1-ri -Z2 -=-1)
^ ^ dx 2 dx ' ^ * dy 2 dy '
.7 \/
^T being a quantity which may be positive or zero but not negative,
given at every point of space.
Finally, let 8 TT Q represent the triple integral
8'TT q = (a2 + 1>* + c2) dx dy dz, (9)
extended over a space bounded by surfaces, for each of which either
V = constant,
or la -\-mb-\-nc = Kl -=- + Km-^- + Kn-^- = q, (10)
dm dy dz
where the value of q is given at every point of the surface ; then, if
a, 6, c be supposed to vary in any manner, subject to the above
conditions, the value of Q will be a unique minimum, when
dV dV dV
a = A-r-> o = JK.-^-> c — K-^- •
due dy dz
Proof.
If we put for the general values of a, b, c,
then, by substituting these values in equations (5) and (7), we find
that u, v, w satisfy the general solenoidal condition
. du dv dw
(!) T" + -J- + T = °-
dx dy dz
\s
We also find, by equations (6) and (8), that at the surfaces of
discontinuity the values of %, vlf w± and u29 v2, w2 satisfy the
superficial solenoidal condition
(2) I(ul—u2) + m(v1 — v2) + n(w1^w2) = 0.
The quantities u, v, w, therefore, satisfy at every point the sole
noidal conditions as stated in the preceding lemma.
98.] UNIQUE MINIMUM OF Q. 105
We may now express Q in terms of u, v, w and V,
~TT W =
The last term of Q may be written 2 Jf, where ^f is the quantity
considered in the lemma, and which we proved to be zero when the
space is bounded by surfaces, each of which is either equipotential
or satisfies the condition of equation (10), which may be written
(4) lu + mv + nw = 0.
Q is therefore reduced to the sum of the first and second terms.
In each of these terms the quantity under the sign of integration
consists of the sum of three squares, and is therefore essentially
positive or zero. Hence the result of integration can only be
positive or zero.
Let us suppose the function V known, and let us find what values
of u, v, w will make Q a minimum.
If we assume that at every point u = 0, v = 0, and w — 0, these
values fulfil the solenoidal conditions, and the second term of Q
is zero, and Q is then a minimum as regards the variation of
«, v, w.
For if any of these quantities had at any point values differing
from zero, the second term of Q would have a positive value, and
Q would be greater than in the case which we have assumed.
But if u = 0, v = 0, and w = 0, then
dx dy dz
Hence these values of a, 3, c make Q a minimum.
But the values of #, 6, c, as expressed in equations (12), are
perfectly general, and include all values of these quantities con
sistent with the conditions of the theorem. Hence, no other values
of a, b, c can make Q a minimum.
Again, Q is a quantity essentially positive, and therefore Q is
always capable of a minimum value by the variation of ay b, c.
Hence the values of a, b, c which make Q a minimum must have
a real existence. It does not follow that our mathematical methods
are sufficiently powerful to determine them.
Corollary I. If a, b, c and K are given at every point of space,
and if we write
106 GENERAL THEOREMS. [98.
(12) a = £% + *, > = K% + *, c = Kd^ + w,
with the condition (1)
du dv dw
~d^ + d^ + ~d~z^ °'
then 7, u, v, w can be found without ambiguity from these four
equations.
Corollary II. The general characteristic equation
d dV d dV Cl
where Fis a finite quantity of single value whose first derivatives
are finite and continuous except at the surface S, and at that surface
fulfil the superficial characteristic
dy
can be satisfied by one value of 7, and by one only, in the following
cases.
Case 1 . When the equations apply to the space within any closed
surface at every point of which 7 = C.
For we have proved that in this case #, b, c have real and unique
values which determine the first derivatives of 7, and hence, if
different values of 7 exist, they can only differ by a constant. But
at the surface 7 is given equal to (7, and therefore 7 is determinate
throughout the space.
As a particular case, let us suppose a space within which p = 0
bounded by a closed surface at which 7=C. The characteristic
equations are satisfied by making V— C for every point within the
space, and therefore V— C is the only solution of the equations.
Case 2. When the equations apply to the space within any closed
surface at every point of which 7 is given.
For if in this case the characteristic equations could be satisfied
by two different values of V, say 7 and 7', put U = 7— 7', then
subtracting the characteristic equation in 7' from that in 7t we
find a characteristic equation in U. At the closed surface £7=0
because at the surface 7 = V, and within the surface the density
is zero because p = p'. Hence, by Case 1, U= 0 throughout the
enclosed space, and therefore 7 = 7' throughout this space.
99-] APPLICATION OF THOMSONS THEOREM. 107
Case 3. When the equations apply to a space bounded by a
closed surface consisting of two parts, in one of which V is given at
every point, and in the other
r,dV dV dV
Kl-j- +Km -=- +Kn -=- = a,
dx dy dz
where q is given at every point.
For if there are two values of F9 let U' represent, as before, their
difference, then we shall have the equation fulfilled within a closed
surface consisting of two parts, in one of which U'= 0, and in the
other JU' dU' dU'
I — h m -= h n -j— = 0 ;
ax dy dz
and since U'= 0 satisfies the equation it is the only solution, and
therefore there is but one value of V possible.
Note. — The function V in this theorem is restricted to one value
at each point of space. If multiple values are admitted, then,
if the space considered is a cyclic space, the equations may be
satisfied by values of V containing terms with multiple values.
Examples of this will occur in Electromagnetism.
99.] To apply this theorem to determine the distribution of
electricity in an electrified system, we must make K •= 1 throughout
the space occupied by air, and K=& throughout the space occupied
by conductors. If any part of the space is occupied by dielectrics
whose inductive capacity differs from that of air, we must make K
in that part of the space equal to the specific inductive capacity.
The value of F, determined so as to fulfil these conditions, will
be the only possible value of the potential in the given system.
Green's Theorem shews that the quantity Q, when it has its
minimum value corresponding to a given distribution of electricity,
represents the potential energy of that distribution of electricity.
See Art. 100, equation (11).
In the form in which Q is expressed as the result of integration
over every part of the field, it indicates that the energy due to the
electrification of the bodies in the field may be considered as the
result of the summation of a certain quantity which exists in every
part of the field where electrical force is in action, whether elec
trification be present or not in that part of the field.
The mathematical method, therefore, in which Q, the symbol
of electrical energy, is made an object of study, instead of p, the
symbol of electricity itself, corresponds to the method of physical
speculation, in which we look for the seat of electrical action in
108 GENERAL THEOREMS. [lOO.
every part of the field, instead of confining our attention to the
electrified bodies.
The fact that Q attains a minimum value when the components
of the electric force are expressed in terms of the first derivatives
of a potential, shews that, if it were possible for the electric force
to be distributed in any other manner, a mechanical force would
be brought into play tending to bring the distribution of force
into its actual state. The actual state of the electric field is
therefore a state of stable equilibrium, considered with reference
to all variations of that state consistent with the actual distribution
of free electricity.
Green's Theorem.
100.] The following remarkable theorem was given by George
Green in his essay ' On the Application of Mathematics to Electricity
and Magnetism.'
I have made use of the coefficient K, introduced by Thomson, to
give greater generality to the statement, and we shall find as we
proceed that the theorem may be modified so as to apply to the
most general constitution of crystallized media.
We shall suppose that U and V are two functions of #, y, z,
which, with their first derivatives, are finite and continuous within
the space bounded by the closed surface S.
We shall also put for conciseness
d vdU d ^dU d ^dU
~r K~^- + -7~K^r + ^-K-r = —4Trp, (1)
das dx dy dy dz dz
d Vd7 d ^dV d ^dV
and —K — + — K -=- + —K -—=—4770', (2)
dy dx dy dy dz dz ^ '
where K is a real quantity, given for each point of space, which
may be positive or zero but not negative. The quantities p and
p correspond to volume-densities in the theory of potentials, but
in this investigation they are -to be considered simply as ab
breviations for the functions of U and V to which they are here
equated.
In the same way we may put
^
~r +mK— + nK-j- =4770-, (3)
dx dy dz
and lK^+mK^+nK^±*]*&. (4)
dx dy dz
where I, m, n are the direction-cosines of the normal drawn inwards
ioo.] GREEN'S THEOREM. 109
from the surface S. The quantities a- and <r' correspond to super
ficial densities, but at present we must consider them as defined by
the above equations.
Green's Theorem is obtained by integrating by parts the ex
pression
TUT r ^ , (IA
4 TT M = IKi-j--^- + -J--J- + -r-r)dxdydz (o)
JJJ \dx dx dy dy dz dz'
throughout the space within the surface S.
If we consider -j- as a component of a force whose potential is T3
and K — as a component of a flux, the expression will give the
work done by the force on the flux.
If we apply the method of integration by parts, we find
or
(7)
In precisely the same manner by exchanging V and T, we should
find rr rrr
4irM=+ 4:TTcrUdS + 1 1 / 47rpc/ dxdydz. (8)
The statement of Green's Theorem is that these three expressions
for M are identical, or that
M = / 1 vr 7dS+ I II p Vdx dydz = / <rUdS + p Udx dy dz
JJ JJJ JJ JJJ
dx ' dy dy dz dz'
Correction of Green's Theorem for Cyclosis.
There are cases in which the resultant force at any point of a
certain region fulfils the ordinary condition of having a potential,
while the potential itself is a many-valued function of the coor
dinates. For instance, if
we find Frrtan-1-, a many -valued function of x and y, the
x
values of V forming an arithmetical series whose common difference
110 GENERAL THEOREMS. [lOO.
is 2 TT, and in order to define which of these is to be taken in
any particular case we must make some restriction as to the line
along which we are to integrate the force from the point where
V = 0 to the required point.
In this case the region in which the condition of having a
potential is fulfilled is the cyclic region surrounding the axis of z,
this axis being a line in which the forces are infinite and therefore
not itself included in the region.
The part of the infinite plane of xz for which x is positive may
be taken as a diaphragm of this cyclic region. If we begin at
a point close to the positive side of this diaphragm, and integrate
along a line which is restricted from passing through the diaphragm,
the line-integral will be restricted to that value of V which is
positive but less than 2 IT.
Let us now suppose that the region bounded by the closed surface
S in Green's Theorem is a cyclic region of any number of cycles,
and that the function V is a many-valued function having any
number of cyclic constants.
dV dV dV
The quantities -=— > -r=— > and -=— will have definite values at all
ax dy dz
points within S, so that the volume-integral
f— — — — ——\
\dx dx dy dy dz dz'
«/ t/
has a definite value, a and p have also definite values, so that if U
is a single valued function, the expression
has also a definite value.
The expression involving V has no definite value as it stands,
for Fis a many- valued function, and any expression containing it
is many-valued unless some rule be given whereby we are directed
to select one of the many values of V at each point of the region.
To make the value of V definite in a region of n cycles, we must
conceive n diaphragms or surfaces, each of which completely shuts
one of the channels of communication between the parts of the
cyclic region. Each of these diaphragms reduces the number of
cycles by unity, and when n of them are drawn the region is still
a connected region but acyclic, so that we can pass from any one
point to any other without cutting a surface, but only by recon-
cileable paths.
100.] INTERPRETATION OF GREENES THEOREM. Ill
Let $! be the first of these diaphragms, and let the line-integral
of the force for a line drawn in the acyclic space from a point
on the positive side of this surface to the contiguous point on
the negative side be KJ , then ^ is the first cyclic constant.
Let the other diaphragms, and their corresponding cyclic con
stants, be distinguished by suffixes from 1 to n, then, since the
region is rendered acyclic by these diaphragms, we may apply to
it the theorem in its original form.
We thus obtain for the complete expression for the first member
of the equation
The addition of these terms to the expression of Green's Theorem,
in the case of many- valued functions, was first shewn to be necessary
by Helmholtz*, and was first applied to the theorem by Thomson.
Physical Interpretation of Green's Theorem.
The expressions a- (IS and pdxdydz denote the quantities of
electricity existing on an element of the surface S and in an
element of volume respectively. We may therefore write for either
of these quantities the symbol e, denoting a quantity of electricity.
We shall then express Green's Theorem as follows —
where we have two systems of electrified bodies, e standing in
succession for elt e.2, &c., any portions of the electrification of the
first system, and Y denoting the potential at any point due to all
these portions, while e' stands in succession for e^, e.2', &c., portions
of the second system, and V denotes the potential at any point
due to the second system.
Hence Ve' denotes the product of a quantity of electricity at a
point belonging to the second system into the potential at that
point due to the first system, and 2 ( Ye'} denotes the sum of all
such quantities, or in other words, 2 ( Ye'} represents that part of
the energy of the whole electrified system which is due to the
action of the second system on the first.
In the same way 2 ( Y'e) represents that part of the energy of
* ' Ueber Integrate der Hydrodynamischen Gleichungen welche den Wirbelbe-
wegungen entsprechen,' Crelle, 1858. Translated by Tait in Phil. Mag., 1867, (»").
t ' On Vortex Motion,' Trans. It. 8. Edin., xxv. part i. p. 241 (1868).
112 GENERAL THEOREMS. [lOO.
the whole system which is due to the action of the first system on
the second.
If we define V as 2 (-), where r is the distance of the quantity e
of electricity from the given point, then the equality between these
two values of M may be obtained as follows, without Green's
Theorem —
This mode of regarding the question belongs to what we have
called the direct method, in which we begin by considering certain
portions of electricity, placed at certain points of space, and acting
on one another in a way depending on the distances between these
points, no account being taken of any intervening medium, or of
any action supposed to take place in the intervening space.
Green's Theorem, on the other hand, belongs essentially to what
we have called the inverse method. The potential is not supposed
to arise from the electrification by a process of summation, but
the electrification is supposed to be deduced from a perfectly
arbitrary function called the potential by a process of differen
tiation.
In the direct method, the equation is a simple extension of the
law that when any force acts directly between two bodies, action
and reaction are equal and opposite.
In the inverse method the two quantities are not proved directly
to be equal, but each is proved equal to a third quantity, a triple
integral which we must endeavour to interpret.
If we write R for the resultant electromotive force due to the
potential V, and 13 m> n for the direction-cosines of R, then, by
Art. 71,
dV -D1 dV „ dV „
-- =-- = EL -- =- = Em. -- 7- = En.
dx dy dz
If we also write E' for the force due to the second system, and
I'j m', ri for its direction-cosines,
ar vr AV dv ,
-- ^ — = K i . --- $ — = K m , -- =— = K n ;
dx dy dz
and the quantity M may be written
M = JL jjf(KRK cos e) dx dy dz, (10)
ioi.] GREEN'S FUNCTION. 113
where cos e = IV -f mm' + nri ',
e being the angle between the directions of R and Iff.
Now if K is what we have called the coefficient of electric
inductive capacity, thenJOZ will be the electric displacement due
to the electromotive force R, and the product \KRRf cose will
represent the work done by the force Rf on account of the dis
placement caused by the force R, or in other words, the amount
of intrinsic energy in that part of the field due to the mutual
action of R and Rf.
We therefore conclude that the physical interpretation of Green's
theorem is as follows :
If the energy which is known to exist in an electrified system
is due to actions which take place in all parts of the field, and
not to direct action at a distance between the electrified bodies,
then that part of the intrinsic energy of any part of the field
upon which the mutual action of two electrified systems depends
is KRRf cos e per unit of volume.
The energy of an electrified system due to its action on itself is,
by Art, 85, 4S(*F),
which is by Green's theorem, putting U — F,
and this is the unique minimum value of the integral considered
in Thomson's theorem.
Green's Function.
101.] Let a closed surface S be maintained at potential zero.
Let P and Q be two points on the positive side of the surface S
(we may suppose either the inside or the outside positive), and
let a small body charged with unit of electricity be placed at P;
the potential at the point Q will consist of two parts, of which one
is due to the direct action of the electricity on P, while the other
is due to the action of the electricity induced on S by P. The
latter part of the potential is called Green's Function, and is
denoted by Gpq.
This quantity is a function of the positions of the two points
P and Q, the form of which depends on that of the surface S. It
has been determined in the case in which 8 is a sphere, and in
a very few other cases. It denotes the potential at Q due to the
electricity induced on S by unit of electricity at P.
VOL. I. I
114 GENERAL THEOREMS. [lOI.
The actual potential at any point Q due to the electricity at P
and on 8 is
where rpq denotes the distance between P and Q.
At the surface S and at all points on the negative side of S9 the
potential is zero, therefore i
pa ~~ ~~ \ /
where the suffix a indicates that a point A on the surface 8 is taken
instead of Q.
Let <rpa? denote the surface-density induced by P at a point A'
of the surface 8, then, since Gpq is the potential at Q due to the
superficial distribution,
where dS' is an element of the surface 8 at A', and the integration
is to be extended over the whole surface 8.
But if unit of electricity had been placed at Q, we should have
had by equation (1), i
where vqa is the density induced by Q on an element dS at A, and
^ is the distance between A and A'. Substituting this value of
- in the expression for Gpq, we find
Since this expression is not altered by changing p into g and
into we find that g _ ^ . ,fiv
**»• ~ ' VTqp * (6)
a result which we have already shewn to be necessary in Art. 88,
but which we now see to be deducible from the mathematical process
by which Green's function may be calculated.
If we assume any distribution of electricity whatever, and place
in the field a point charged with unit of electricity, and if the
surface of potential zero completely separates the point from the
assumed distribution, then if we take this surface for the surface St
and the point for P, Green's function, for any point on the same
side of the surface as P, will be the potential of the assumed dis
tribution on the other side of the surface. In this way we may
construct any number of cases in which Green's function can be
102.] MINIMUM VALUE OF Q. 116
found for a particular position of P. To find the form of the
function when the form of the surface is given and the position
of P is arbitrary, is a problem of far greater difficulty, though,
as we have proved, it is mathematically possible.
Let us suppose the problem solved, and that the point P is
taken within the surface. Then for all external points the potential
of the superficial distribution is equal and opposite to that of P.
The superficial distribution is therefore centrobaric *, and its action
on all external points is the same as that of a unit of negative
electricity placed at P.
Method of Approximating to the Values of Coefficients of Capacity, fyc.
102.] Let a region be completely bounded by a number of
surfaces £0, S^ S2, &c., and let K be a quantity, positive or zero
but not negative, given at every point of this region. Let V
be a function subject to the conditions that its values at the
surfaces S19 S2t &c. are the constant quantities Clt C2, &c., and that
at the surface SQ dV /^
^=°'
where v is a normal to the surface £0. Then the integral
taken over the whole region, has a unique minimum when V satisfies
the equation d ^dV d ^dV d ^dV , .
— A -7 — \--j-K— — \- -=- K -=- = 0 (3)
dx dx dy dy dz dz
throughout the region, as well as the original conditions.
We have already shewn that a function V exists which fulfils the
conditions (1) and (3), and that it is determinate in value. We
have next to shew that of all functions fulfilling the surface-con
ditions it makes Q a minimum.
Let F0 be the function which satisfies (1) and (3), and let
F=F0+*7 (4)
be a function which satisfies (1).
It follows from this that at the surfaces S19 82, &c. U= 0.
The value of Q becomes
Thomson and Tait's Natural Philosophy, § 526.
I 2
116 GENERAL THEOREMS. [lO2.
Let us confine our attention to the last of these three groups
of terms, merely observing that the other groups are essentially
positive. By Green's theorem
/7F" fJIT flV //77 HV /7/7v CC HV
/ttr Q U'U ttr Q ClU (tr Q au \ ., , , ///rr/"°7^
V dx dx dy dy dz dz JJ dv
_[[fu( — K^+—K^ + ~K^}d'xddz' (6)
J JJ ^da) dx dy dy dz dz '
fff
dy dy
the first integral of the second member being extended over the
surface of the region and the second throughout the enclosed space.
But on the surfaces S^ S2> &c. U= 0, so that these contribute
nothing to the surface-integral.
Again, on the surface SQJ -~- = 0, so that this surface contributes
Cv V
nothing to the integral. Hence the surface-integral is zero.
The quantity within brackets in the volume-integral also dis
appears by equation (3), so that the volume-integral is also zero.
Hence Q is reduced to
Both these quantities are essentially positive, and therefore the
minimum value of Q is when
— -^-^-o (8)
dx dy dz
or when U is a constant. But at the surfaces S, &c. U = 0. Hence
U = 0 everywhere, and F0 gives the unique minimum value of Q.
Calculation of a Superior Limit of the Coefficients of Capacity.
The quantity Q in its minimum form can be expressed by means
of Green's theorem in terms of F19 F2, &c., the potentials of Slt S2J
and JE19 U2, &c., the charges of these surfaces,
q = ^(r1M1+rtJBt+&0.)i (9)
or, making use of the coefficients of capacity and induction as defined
in Article 87,
Q = i(^i2^ii+^2^2 + &c.)+F1r2^12+&c. (10)
The accurate determination of the coefficients q is in general
difficult, involving the solution of the general equation of statical
electricity, but we make use of the theorem we have proved to
determine a superior limit to the value of any of these coefficients.
102.] METHOD OF APPEOXIMATION. 117
To determine a superior limit to the coefficient of capacity qu ,
make V± = 1, and V2J V^ &c. each equal to zero, and then take
any function V which shall have the value 1 at S13 and the value 0
at the other surfaces.
From this trial value of V calculate Q by direct integration,
and let the value thus found be Q'. We know that Q' is not less
than the absolute minimum value Q, which in this case is \ qn.
Hence qu is not greater than 2 (JX. (11)
If we happen to have chosen the right value of the function
F, then qn = 2 Q', but if the function we have chosen differs
slightly from the true form, then, since Q is a minimum, Q' will
still be a close approximation to the true value.
Superior Limit of the Coefficients of Potential.
We may also determine a superior limit to the coefficients of
potential denned in Article 86 by means of the minimum value
of the quantity Q in Article 98, expressed in terms of a, b, c.
By Thomson's theorem, if within a certain region bounded by the
surfaces S0, £15 &c. the quantities a, 6, c are subject to the condition
da clb dc
and if la + ml + nc = q (1 3)
be given all over the surface, where I, m, n are the direction-cosines
of the normal, then the integral
e = tiff's: («*+**+«*)*"&'*' (14)
is an absolute and unique minimum when
dV ^clV ^dV , .
a — K-^-t l = K^-> c = K-j-> (15)
dx ay dz
When the minimum is attained Q is evidently the same quantity
which we had before.
If therefore we can find any form for a, b, c which satisfies the
condition (12) and at the same time makes
JS.2&c.', (16)
and if Q" be the value of Q calculated by (14) from these values of
a, b, c, then Q" is not less than
(17)
118 GENERAL THEOREMS. [lO2.
If we take the case in which one of the surfaces, say S2, sur
rounds the rest at an infinite distance, we have the ordinary case
of conductors in an infinite region ; and if we make Ez = — U19 and
E — 0 for all the other surfaces, we have F~2 = 0 at infinity, and
2 Q"
jpn is not greater than — ^- •
In the very important case in which the electrical action is
entirely between two conducting surfaces S1 and $2, of which S2
completely surrounds ^ and is kept at potential zero, we have
El = — fi2 and qnpu = 1.
Hence in this case we have
XT
qu not less than -^-f \ (18)
and we had before ,. -,
qn not greater than 2 Q ; (19)
so that we conclude that the true value of qll9 the capacity of the
internal conductor, lies between these values.
This method of finding superior and inferior limits to the values
of these coefficients was suggested by a memoir ' On the Theory
of Resonance/ by the Hon. J. W. Strutt, Phil. Trans., 1871. See
Art. 308.
CHAPTER V.
MECHANICAL ACTION BETWEEN ELECTRIFIED BODIES.
103.] Let Y — C be any closed equipotential surface, C being
a particular value of a function T, the form of which we suppose
known at every point of space. Let the value of V on the outside
of this surface be T\, and on the inside 7.2. Then, by Poisson's
equation
(1)
we can determine the density p± at every point on the outside, and
the density p2 at every point on the inside of the surface. We shall
call the whole electrified system thus explored on the outside U13
and that on the inside E2. The actual value of Y arises from the
combined action of both these systems.
Let R be the total resultant force at any point arising from
the action of El and K2, R is everywhere normal to the equi
potential surface passing through the point.
Now let us suppose that on the equipotential surface Y = C
electricity is distributed so that at any point of the surface at
which the resultant force due to E± and K2 reckoned outwards
is Rj the surface-density is a, with the condition
R = 4 T: o- ; (2)
and let us call this superficial distribution the electrified surface S,
then we can prove the following theorem relating to the action of
this electrified surface.
If any equipotential surface belonging to a given electrified
system be coated with electricity, so that at each point the surface-
density o- = — , where R is the resultant force, due to the original
47T
electrical system, acting outwards from that point of the surface,
then the potential due to the electrified surface at any point on
120 ELECTRIC ATTRACTION. [103.
the outside of that surface will be equal to the potential at the
same point due to that part of the original system which was on
the inside of the surface, and the potential due to the electrified
surface at any point on the inside added to that due to the part of
the original system on the outside will be equal to C, the potential
of the surface.
For let us alter the original system as follows :
Let us leave everything the same on the outside of the surface,
but on the inside let us make T2 everywhere equal to C, and let us
do away with the electrified system Ez on the inside of the surface,
and substitute for it a surface-density a at every point of the
surface S, such that It — ^n a. (3)
Then this new arrangement will satisfy the characteristics of V at
every point.
For on the outside of the surface both the distribution of elec
tricity and the value of V are unaltered, therefore, since V originally
satisfied Laplace's equation, it will still satisfy it.
On the inside V is constant and p zero. These values of V and p
also satisfy the characteristic equations.
At the surface itself, if V-^ is the potential at any point on the
outside and V^ that on the inside, then, if I, m, n are the direction-
cosines of the normal to the surface reckoned outwards,
,dV, dV, dV, (A.
I i +m — i+^-—i = -^ = -471(7 ; (4)
dx dy dz
and on the inside the derivatives of V vanish, so that the superficial
characteristic
d72. fdV, dV^ fdV, dV^
--T^)+^(-7-1--r-)+^(-TJ-- --7-^ + 4 w<r=0 (5)
dx dx ' ^ dy dy ' ^ dz dz '
J u
is satisfied at every point of the surface.
Hence the new distribution of potential, in which it has the
old value on the outside of the surface and a constant value on
the inside, is consistent with the new distribution of 'electricity,
in which the electricity in the space within the surface is removed
and a distribution of electricity on the surface is substituted for
it. Also, since the original value of V-^ vanishes at infinity, the
new value, which is the same outside the surface, also fulfils this
condition, and therefore the new value of V is the sole and only
value of V belonging to the new arrangement of electricity.
1 04.] EQUIVALENT ELECTRIFIED SURFACE. 121
On the Mechanical Action and Reaction of the Systems E1 and E2.
104.] If we now suppose the equipotential surface V — C to
become rigid and capable of sustaining the action of forces, we
may prove the following theorem.
If on every element dS of an equipotential surface a force
— R2 dS be made to act in the direction of the normal reckoned
Sir
outwards, where R is the ' electrical resultant force ' along the
normal, then the total statical effect of these forces on the
surface considered as a rigid shell will be the same as the total
statical effect of the electrical action of the electrified system E±
outside the shell on the electrified system E2 inside the shell, the
parts of the interior system E2 being supposed rigidly connected
together.
We have seen that the action of the electrified surface in the last
theorem on any external point was equal to that of the internal
system E2J and, since action and reaction are equal and opposite,
the action of any external electrified body on the electrified surface,
considered as a rigid system, is equal to that on the internal system
E2. Hence the statical action of the external system E-± on the
electrified surface is equal in all respects to the action of E1 on the
internal system E.2.
But at any point just outside the electrified surface the resultant
force is R in a direction normal to the surface, and reckoned positive
when it acts outwards. The resultant inside the surface is zero,
therefore, by Art. 79, the resultant force acting on the element
dS of the electrified surface is \RadS, where cr is the surface-
density.
Substituting the value of a- in terms of R from equation (2), and
denoting by p dS the resultant force on the electricity spread over
the element dS, we find
—
O7T
This force always acts along the normal and outwards, whether
R be positive or negative, and may be considered as equal to a
pressure p= — R2 acting on the surface from within, or to a tension
STT
of the same numerical value acting from without.
* See Sir W. Thomson ' On the Attractions of Conducting and Non-conducting
Electrified Bodies,' Cambridge Mathematical Journal, May 1843, and Reprint,
Art. VII, § 147.
122 ELECTKIC ATTRACTION.
Now R is the resultant due to the combined action of the
external system E± and the electrification of the surface S. Hence
the effect of the pressure/? on each element of the inside of the surface
considered as a rigid body is equivalent to this combined action.
But the actions of the different parts of the surface on each other
form a system in equilibrium, therefore the effect of the pressure p on
the rigid shell is equivalent in all respects to the electric attraction
of EI on the shell, and this, as we have before shewn, is equivalent
to the electric attraction of E-^ on E2 considered as a rigid system.
If we had supposed the pressure p to act on the outside of the
shell, the resultant effect would have been equal and opposite, that
is, it would have been statically equivalent to the action of the
internal system E2 on the external system E^.
Let us now take the case of two electrified systems E^ and
E2t such that two equipotential surfaces F = Cl and F = C2, which
we shall call Sl and S2 respectively, can be described so that E^ is
exterior to S19 and Sl surrounds S2, and E% lies within S2.
Then if Rl and R2 represent the resultant force at any point of
Sl and S2 respectively, and if we make
the mechanical action between El and E2 is equivalent to that
between the shells ^ and S2, supposing every point of S1 pressed
inwards, that is, towards S2 with a pressure plt and every point of
S2 pressed outwards, that is, towards S1 with a pressure p2 .
105.] According to the theory of action at a distance the action
between E^ and E2 is really made up of a system of forces acting in
straight lines between the electricity in El and that in H29 and the
actual mechanical effect is in complete accordance with this theory.
There is, however, another point of view from which we may
examine the action between El and E2. When we see one body
acting on another at a distance, before we assume that the one
acts directly on the other we generally inquire whether there is
any material connexion between the two bodies, and if we find
strings, or rods, or framework of any kind, capable of accounting
for the observed action between the bodies, we prefer to explain
the action by means of the intermediate connexions, rather than
admit the notion of direct action at a distance.
Thus when two particles are connected by a straight or curved
rod, the action between the particles is always along the line joining
them, but we account for this action by means of a system of
106.] INTERNAL FORCES. 123
internal forces in the substance of the rod. The existence of these
internal forces is deduced entirely from observation of the effect
of external forces on the rod, and the internal forces themselves
are generally assumed to be the resultants of forces which act
between particles of the rod. Thus the observed action between
two distant particles is, in this instance, removed from the class
of direct actions at a distance by referring it to the intervention
of the rod ; the action of the rod is explained by the existence
of internal forces in its substance ; and the internal forces are
explained by means of forces assumed to act between the particles
of which the rod is composed, that is, between bodies at distances
which though small must be finite.
The observed action at a considerable distance is therefore ex
plained by means of a great number of forces acting between
bodies at very small distances, for which we are as little able to
account as for the action at any distance however great.
Nevertheless, the consideration of the phenomenon, as explained
in this way, leads us to investigate the properties of the rod, and
to form a theory of elasticity \\hich we should have overlooked
if we had been satisfied with the explanation by action at a distance,
106.] Let us now examine the consequence of assuming that the
action between electrified bodies can be explained by the inter
mediate action of the medium between them, and let us ascertain
what properties of the medium will account for the observed action.
We have first to determine the internal forces in the medium,
and afterwards to account for them if possible.
In order to determine the internal forces in any case we proceed
as follows :
Let the system M be in equilibrium under the action of the
system of external forces F. Divide M by an imaginary surface
into two parts, M^ and M.2, and let the systems of external forces
acting on these parts respectively be FL and F.2. Also let the
internal forces acting on Ml in consequence of its connexion with
M.2 be called the system /.
Then, since Ml is in equilibrium under the action of Fl and /,
it follows that / is statically equivalent to Fl reversed.
In the case of the electrical action between two electrified systems
E1 and E.2> we described two closed equipotential surfaces entirely
surrounding E2 and cutting it off" from E13 and we found that the
application of a certain normal pressure at every point of the inner
side of the inner surface, and on the outer side of the outer surface,
124 ELECTRIC ATTRACTION. [lo6.
would, if these surfaces were each rigid, act on the outer surface
with a resultant equal to that of the electrical forces on the outer
system Ult and on the inner surface with a resultant equal to that
of the electrical forces on the inner system.
Let us now consider the space between the surfaces, and let us
suppose that at every point of this space there is a tension in the
direction of R and equal to — R* per unit of area. This tension
07T
will act on the two surfaces in the same way as the pressures on
the other side of the surfaces, and will therefore account for the
action between E^ and E2, so far as it depends on the internal force
in the space between S± and S2.
Let us next investigate the equilibrium of a portion of the shell
bounded by these surfaces and separated from the rest by a surface
everywhere perpendicular to the equipotential surfaces. We may
suppose this surface generated by describing any closed curve on
8lf and drawing from every point of this curve lines of force till
they meet S2 .
The figure we have to consider is therefore bounded by the two
equipotential surfaces 8l and S2, and by a surface through which
there is no induction, which we may call S0.
Let us first suppose that the area of the closed curve on 81 is very
small, call it dSlt and that C2 = Cl + dTr.
The portion of space thus bounded may be regarded as an element
of volume. If v is the normal to the equipotential surface, and
dS the element of that surface, then the volume of this element
is ultimately dSdv.
The induction through dSl is RdSlt and since there is no in
duction through S0 , and no free electricity within the space con
sidered, the induction through the opposite surface dS2 will be
equal and opposite, considered with reference to the space within
the closed surface.
There will therefore be a quantity of electricity
%!P.T15*«i
on the first equipotential surface, and a quantity
*2 = l^R*d8*
on the second equipotential surface, with the condition
= 0.
1O6.] RESULTANT OF ELECTRIC TENSIONS. 125
Let us next consider the resultant force due to the action of the
electrified systems on these small electrified surfaces.
The potential within/ the surface Sl is constant and equal to Clt
and ^without the surface S2 it is constant and equal to C.2. In the
shell between these surfaces it is continuous from Q to C2 .
Hence the resultant force is zero except within the shell.
The electrified surface of the shell itself will be acted on by forces
which are the arithmetical means of the forces just within and just
without the surface, that is, in this case, since the resultant force
outside is zero, the force acting on the superficial electrification is
one-half of the resultant force just within the surface.
Hence, if XdSdv be the total moving force resolved parallel
to x, due to the electrical action on both the electrified surfaces of
the element dSdv,
where the suffixes denote that the derivatives of Vare to be taken
at dS1 and dS.2 respectively.
Let I, m, n be the direction-cosines of F, the normal to the
equipotential surface, then making
dx — I dv, dy — m dv, and dz = n dv,
. (^ fl
-— ) = (-J-) +(£-7-5 +m -=— r + n -j— ~) dv + &c. ;
dx'z dx'-L \ dx2 dxdy '
and since e2 = — el, we may write the value of X
XdSdv = i e, -j- (l— +m-j- +n -j-} dv.
1 dx ^ dx dy dz '
But e, = -- -EdS and (l*— +m(-j- +n-j-) = —R;
v '
dx d dz
therefore XdSdv = — R~
Sir dx
or, if we write
dr
v i dp v i dp 7 i dP .
then *=i^> Y=*fy' Z=^>
or the force in any direction on the element arising from the action
of the electrified system on the two electrified surfaces of the
element is equal to half the rate of increase of p in that direction
multiplied by the volume of the element.
126 ELECTRIC ATTRACTION. [106.
This result is the same if we substitute for the forces acting on
the electrified surfaces an imaginary force whose potential is — \p^
acting on the whole volume of the element and soliciting it to
move so as to increase \p.
If we now return to the case of a figure of finite size, bounded
by the equipotential surfaces Sl and S2 and by the surface of no
induction SQ9 we may divide the whole space into elements by a
series of equipotential surfaces and two series of surfaces of no
induction. The charges of electricity on those faces of the elements
which are in contact will be equal and opposite, so that the total
effect will be that due to the electrical forces acting on the charges
on the surfaces Sl and $2, and by what we have proved this will be
the same as the action on the whole volume of the figure due to a
system of forces whose potential is — \p.
But we have already shewn that these electrical forces are
equivalent to a tension p applied at all points of the surfaces S1
and S.2. Hence the effect of this tension is to pull the figure in
the direction in which p increases. The figure therefore cannot be
in equilibrium unless some other forces act on it.
Now we know that if a hydrostatic pressure p is applied at every
point of the surface of any closed figure, the effect is equal to
that of a system of forces acting on the whole volume of the figure
and having a potential p. In this case the figure is pushed in
the direction in which p diminishes.
We can now arrange matters so that the figure shall be in
equilibrium.
At every point of the two equipotential surfaces Sl and $2, let
a tension — p be applied, and at every point of the surface of no
induction $0 let a pressure = p be applied. These forces will keep
the figure in equilibrium.
For the tension p may be considered as a pressure p combined
with a tension 2 p. We have then a hydrostatic pressure^? acting
at every point of the surface, and a tension 2 p acting on ^ and S2
only.
The effect of the tension 2p at every point of Sj_ and S2 is double
of that which we have just calculated, that is, it is equal to that
of forces whose potential is — p acting on the whole volume of the
figure. The effect of the pressure p acting on the whole surface
is by hydrostatics equal and opposite to that of this system of
forces, and will keep the figure in equilibrium.
107.] We have now determined a system of internal forces in
1 07.] STRESS IN A DIELECTRIC MEDIUM. 127
the medium which is consistent with the phenomena so far as
we have examined them. We have found that in order to account
for the electric attraction between distant bodies without admitting
direct action, we must assume the existence of a tension p at every
point of the medium in the direction of the resultant force R at
that point. In order to account for the equilibrium of the medium
itself we must further suppose that in every direction perpendicular
to R there is a pressure p*.
By establishing the necessity of assuming these internal forces
in the theory of an electric medium, we have advanced a step in
that theory which will not be lost though we should fail in
accounting for these internal forces, or in explaining the mechanism
by which they can be maintained in air, glass, and other dielectric
media.
We have seen that the internal stresses in solid bodies can be
ascertained with precision, though the theories which account for
these stresses by means of molecular forces may still be doubtful.
In the same way we may estimate these internal electrical forces
before we are able to account for them.
In order, however, that it may not appear as if we had no
explanation of these internal forces, we shall shew that on the
ordinary theory they must exist in a shell bounded by two equipo-
tential surfaces, and that the attractions and repulsions of the elec
tricity on the surfaces of the shell are sufficient to account for them.
Let the first surface Sl be electrified so that the surface-density is
and the second surface S2 so that the surface-density is
*2= — ^j
then, if we suppose that the value of V is C± at every point within
S19 and C2 at every point outside of S.2, the value of F between these
surfaces remaining as before, the characteristic equation of Fwill
be satisfied everywhere, and V is therefore the true value of the
potential.
We have already shewn that the outer and inner surfaces of the
shell will be pulled towards each other with a force the value of
which referred to unit of surface is p, or in other words, there is a
tension p in the substance of the shell in the direction of the lines
of force.
* See Faraday, Exp. Res. (1224) and (1297).
128 ELECTRIC ATTRACTION. [108.
If we now conceive the shell divided into two segments by a
surface of no induction, the two parts will experience electrical
forces the resultants of which will tend to separate the parts with
a force equivalent to the resultant force due to a pressure p acting
on every part of the surface of no induction which divides them.
This illustration is to be taken merely as an explanation of what
is meant by the tension and pressure, not as a physical theory to
account for them.
108.] We have next to consider whether these internal forces
are capable of accounting for the observed electrical forces in every
case, as well as in the case where a closed equipotential surface can
be drawn surrounding one of the electrified systems.
The statical theory of internal forces has been investigated by
writers on the theory of elasticity. At present we shall require only
to investigate the effect of an oblique tension or pressure on an
element of surface.
Let p be the value of a tension referred to unit of a surface to
which it is normal, and let there be no tension or pressure in any
direction normal to p. Let the direction-cosines of p be I, m, n.
Let dy dz be an element of surface normal to the axis of x, and let
the effect of the internal force be to urge the parts on the positive
side of this element with a force whose components are
pxx dy dz in the direction of #,
Pxydydz y, and
pxzdy dz z.
From every point of the boundary of the element dy dz let lines
be drawn parallel to the direction of the tension j9, forming a prism
whose axis is in the line of tension, and let this prism be cut by a
plane normal to its axis.
The area of this section will be I dy dx, and the whole tension
upon it will be p I dy d^ and since there is no action on the sides
of the prism, which are normal to jo, the force on the base dy dz
must be equivalent to the force p I dy dd acting in the direction
(I, m, n). Hence the component in the direction of #,
Pxx cty dz = pi2 dy dz ; or
P«* = PP-
Similarly pxy — plm, (1)
pxz = pin.
If we now combine with this tension two tensions p' and p" in
directions (I', m', n') and (l"} m" ', n") respectively, we shall have
IO8.] COMPONENTS OF STRESS. 129
pxy = pirn + p' r m' + p" I" m", (2)
pxz = pln+p'l'n' +p"l"n".
In the case of the electrical tension and pressure the pressures
are numerically equal to the tension at every point, and are in
directions at right angles to the tension and to each other. Hence,
putting p'=p"=-p, (3)
we find j^ = (2^2
(5)
for the action of the combined tension and pressures.
Also, since p = -— R2, where R denotes the resultant force, and
OTT
since El = X, Em = Y, En — Z,
pxx = ±(X*-Y*
, = />... (6)
STT
where X, J", ^ are the components of R, the resultant electromotive
force.
The expressions for the component internal forces on surfaces
normal to y and z may be written down from symmetry.
To determine the conditions of equilibrium of the element dxdydz.
This element is bounded by the six planes perpendicular to the
axes of coordinates passing through the points (a?, y, z) and (x -f dxt
y + dy, z + dz).
The force in the direction of x which acts on the first face dy dz
is — pxxdydz, tending to draw the element towards the negative
side. On the second face dy dz, for which x has the value x -f dx^
the tension pxx has the value
pxx dy dz+ (fa Pxx) <fa dy dz,
and this tension tends to draw the element in the positive direction.
If we next consider the two faces dzdx with respect to the
VOL. I. K
130 ELECTRIC ATTRACTION. [108.
tangential forces urging them in the direction of x, we find the
force on the first face — pyx dz dx> and that on the second
pyx dz dx + (-j- p
Similarly for the faces dx dy, we find that a force — pzx dx dy acts
on the first face, and
pKX dx dy -f (_j-pzx) dx dy dz
on the second in the direction of x.
If £dxdydz denotes the total effect of all these internal forces
acting parallel to the axis of x on the six faces of the element, we find
f dx dy dz = (-j-pxx + -7- pvx + -r pzx) dx dy dz :
dx dy dz '
or, denoting by f the internal force, referred to unit of volume, and
resolved parallel to the axis of x,
£=d_ d_ i_
with similar expressions for r; and £ the component forces in the
other directions *.
Differentiating the values of pxx,pyx, and pzx given in equations
(6), we find
But by Art. 77
,dX dY dZ^
Hence (
Similarly 77 = p7, (10)
C=pZ.
Thus, the resultant of the tensions and pressures which we have
supposed to act upon the surface of the element is a force whose
components are the same as those of the force, which, in the
ordinary theory, is ascribed to the action of electrified bodies on the
electricity within the element.
If, therefore, we admit that there is a medium in which there
is maintained at every point a tension p in the direction of the
* This investigation may be compared with that of the ' equation of continuity
in hydrodynamics,' and with others in which the effect on an element of volume
is deduced from the values of certain quantities at its bounding surface.
no.] FARADAY'S THEORY. 131
resultant electromotive force R> and such that R2 = 8717?, combined
with an equal pressure p in every direction at right angles to the
resultant 7?, then the mechanical effect of these tensions and
pressures on any portion of the medium, however bounded, will be
identical with the mechanical effect of the electrical forces according
to the ordinary theory of direct action at a distance.
109.] This distribution of stress is precisely that to which Fara
day was led in his investigation of induction through dielectrics.
He sums up in the following words : —
'(1297) The direct inductive force, which may be conceived to
be exerted in lines between the two limiting and charged con
ducting surfaces, is accompanied by a lateral or transverse force
equivalent to a dilatation or repulsion of these representative lines
(1224.); or the attracting force which exists amongst the par
ticles of the dielectric in the direction of the induction is ac
companied by a repulsive or a diverging force in the transverse
direction.
'(1298) Induction appears to consist in a certain polarized state
of the particles, into which they are thrown by the electrified body
sustaining the action, the particles assuming positive and negative
points or parts, which are symmetrically arranged with respect
to each other and the inducting surfaces or particles. The state
must be a forced one, for it is originated and sustained only by
force, and sinks to the normal or quiescent state when that force
is removed. It can be continued only in insulators by the same
portion of electricity, because they only can retain this state of the
particles.'
This is an exact account of the conclusions to which we have
been conducted by our mathematical investigation. At every point
of the medium there is a state of stress such that there is tension
along the lines of force and pressure in all directions at right angles
to these lines, the numerical magnitude of the pressure being equal
to that of the tension, and both varying as the square of the
resultant force at the point.
The expression 'electric tension' has been used in various senses
by different writers. I shall always use it to denote the tension
along the lines of force, which, as we have seen, varies from point
to point, and is always proportional to the square of the resultant
force at the point.
110.] The hypothesis that a state of stress of this kind exists
in a fluid dielectric, such as air or turpentine, may at first sight
K 2
132 ELECTRIC ATTRACTION. [ill.
appear at variance with the established principle that at any point
in a fluid the pressures in all directions are equal. But in the
deduction of this principle from a consideration of the mobility
and equilibrium of the parts of the fluid it is taken for granted
that no action such as that which we here suppose to take place
along- the lines of force exists in the fluid. The state of stress
which we have been studying is perfectly consistent with the
mobility and equilibrium of the fluid, for we have seen that, if
any portion of the fluid is devoid of electric charge, it experi
ences no resultant force from the stresses on its surface, however
intense these may be. It is only when a portion of the fluid
becomes charged, that its equilibrium is disturbed by the stresses
on its surface, and we know that in this case it actually tends to
move. Hence the supposed state of stress is not inconsistent with
the equilibrium of a fluid dielectric.
The quantity Q, which was investigated in Thomson's theorem,
Art. 98, may be interpreted as the energy in the medium due to
the distribution of stress. It appears from that theorem that the
distribution of stress which satisfies the ordinary conditions also
makes Q an absolute minimum. Now when the energy is a
minimum for any configuration, that configuration is one of equi
librium, and the equilibrium is stable. Hence the dielectric,
when subjected to the inductive action of electrified bodies, will
of itself take up a state of stress distributed in the way we have
described.
It must be carefully borne in mind that we have made only one
step in the theory of the action of the medium. We have supposed
it to be in a state of stress, but we have not in any way accounted
for this stress, or explained how it is maintained. This step,
however, seems to me to be an important one, as it explains, by
the action of the consecutive parts of the medium, phenomena which
were formerly supposed to be explicable only by direct action at
a distance.
111.] I have not been able to make the next step, namely, to
account by mechanical considerations for these stresses in the
dielectric. I therefore leave the theory at this point, merely
stating what are the other parts of the phenomenon of induction
in dielectrics.
I. Electric Displacement. When induction takes place in a
dielectric a phenomenon takes place which is equivalent to a
displacement of electricity in the direction of the induction. For
III.] ELECTRIC POLARIZATION. 133
instance, in a Leyden jar, of which the inner coating is charged
positively and the outer coating negatively, the displacement in
the substance of the glass is from within outwards.
Any increase of this displacement is equivalent, during the time
of increase, to a current of positive electricity from within outwards,
and any diminution of the displacement is equivalent to a current
in the opposite direction.
The whole quantity of electricity displaced through any area
of a surface fixed in the dielectric is measured by the quantity which
we have already investigated (Art. 75) as the surface-integral of
induction through that area, multiplied by — K, where K is the
specific inductive capacity of the dielectric.
II. Superficial Electrification of the Particles of the Dielectric.
Conceive any portion of the dielectric, large or small, to be separated
(in imagination) from the rest by a closed surface, then we must
suppose that on every elementary portion of this surface there is
an electrification measured by the total displacement of electricity
through that element of surface reckoned Inwards.
In the case of the Leyden jar of which the inner coating is
charged positively, any portion of the glass will have its inner
side charged positively and its outer side negatively. If this
portion be entirely in the interior of the glass, its superficial elec
trification will be neutralized by the opposite electrification of the
parts in contact with it, but if it be in contact with a conducting
body which is incapable of maintaining in itself the inductive state,
the superficial electrification will not be neutralized, but will con
stitute that apparent electrification which is commonly called the
Electrification of the Conductor.
The electrification therefore at the bounding surface of a con
ductor and the surrounding dielectric, which on the old theory
was called the electrification of the conductor, must be called in the
theory of induction the superficial electrification of the surrounding
dielectric.
According to this theory, all electrification is the residual effect
of the polarization of the dielectric. This polarization exists
throughout the interior of the substance, but it is there neutralized
by the juxtaposition of oppositely electrified parts, so that it is only
at the surface of the dielectric that the effects of the electrification
become apparent.
The theory completely accounts for the theorem of Art. 7 7, that
134 ELECTRIC ATTRACTION. [ill.
the total induction through a closed surface is equal to the total
quantity of electricity within the surface multiplied by 4-n". For
what we have called the induction through the surface is simply
the electric displacement multiplied by 47r, and the total displace
ment outwards is necessarily equal to the total electrification within
the surface.
The theory also accounts for the impossibility of communicating
an absolute charge to matter. For every particle of the dielectric
is electrified with equal and opposite charges on its opposite sides,
if it would not be more correct to say that these electrifications are
only the manifestations of a single phenomenon, which we may call
Electric Polarization.
A dielectric medium, when thus polarized, is the seat of electrical
energy, and the energy in unit of volume of the medium is nu
merically equal to the electric tension on unit of area, both quan
tities being equal to half the product of the displacement and the
resultant electromotive force, or
,
where p is the electric tension, & the displacement, <£ the electro
motive force, and K the specific inductive capacity.
If the medium is not a perfect insulator, the state of constraint,
which we call electric polarization, is continually giving way. The
medium yields to the electromotive force, the electric stress is
relaxed, and the potential energy of the state of constraint is con
verted into heat. The rate at which this decay of the state of
polarization takes place depends on the nature of the medium.
In some kinds of glass, days or years may elapse before the polar
ization sinks to half its original value. In copper, this change
may occupy less than the billionth of a second.
We have supposed the medium after being polarized to be simply
left to itself. In the phenomenon called the electric current the
constant passage of electricity through the medium tends to restore
the state of polarization as fast as the conductivity of the medium
allows it to decay. Thus the external agency which maintains the
current is always doing work in restoring the polarization of the
medium, which is continually becoming relaxed, and the potential
energy of this polarization is continually becoming transformed
into heat, so that the final result of the energy expended in main
taining the current is to raise the temperature of the conductor.
CHAPTER VI.
ON POINTS AND LINES OF EQUILIBKIUM.
112.] IF at any point of the electric field the resultant force is
zero, the point is called a Point of equilibrium.
If every point on a certain line is a point of equilibrium, the line
is called a Line of equilibrium.
The conditions that a point shall be a point of equilibrium are
that at that point
d7 dV dV
— - — 0, -=- = 0, -j- = 0.
dx dy dz
At such a point, therefore, the value of V is a maximum, or
a minimum, or is stationary, with respect to variations of the
coordinates. The potential, however, can have a maximum or a
minimum value only at a point charged with positive or with
negative electricity, or throughout a finite space bounded by a
surface electrified positively or negatively. If, therefore, a point
of equilibrium occurs in an unelectrified part of the field it must
be a stationary point, and not a maximum or a minimum.
In fact, the first condition of a maximum or minimum is that
9
dx2 dy*
must be all negative or all positive, if they have finite values.
Now, by Laplace's equation, at a point where there is no elec
trification, the sum of these three quantities is zero, and therefore
this condition cannot be fulfilled.
Instead of investigating the analytical conditions for the cases
in which the components of the force simultaneously vanish, we
shall give a general proof by means of the equipotential surfaces.
If at any point, P, there is a true maximum value of F, then, at
all other points in the immediate neighbourhood of P, the value of
V is less than at P. Hence P will be surrounded by a series of
136 POINTS AND LINES OF EQUILIBRIUM. [113'
closed equipotential surfaces, each outside the one before it, and at
all points of any one of these surfaces the electrical force will be
directed outwards. But we have proved, in Art. 76, that the surface-
integral of the electrical force taken over any closed surface gives
the total electrification within that surface multiplied by 4 IT. Now,
in this case the force is everywhere outwards, so that the surface-
integral is necessarily positive, and therefore there is positive elec
trification within the surface, and, since we may take the surface as
near to P as we please, there is positive electrification at the point P.
In the same way we may prove that if V is a minimum at P,
then P is negatively electrified.
Next, let P be a point of equilibrium in a region devoid of elec
trification, and let us describe a very small closed surface round
P, then, as we have seen, the potential at this surface cannot be
everywhere greater or everywhere less than at P. It must there
fore be greater at some parts of the surface and less at others.
These portions of the surface are bounded by lines in which the
potential is equal to that at P. Along lines drawn from P to
points at which the potential is less than that at P the electrical
force is from P, and along lines drawn to points of greater po
tential the force is towards P. Hence the point P is a point of
stable equilibrium for some displacements, and of unstable equili
brium for other displacements.
113.] To determine the number of the points and lines of equi
librium, let us consider the surface or surfaces for which the
potential is equal to <?, a given quantity. Let us call the regions
in which the potential is less than C the negative regions, and
those in which it is greater than C the positive regions. Let
T0 be the lowest, and V^ the highest potential existing in the
electric field. If we make C = V0, the negative region will in
clude only the electrified point or conductor of lowest potential,
and this is necessarily electrified negatively. The positive region
consists of the rest of space, and since it surrounds the negative
region it is periphractic. See Art. 18.
If we now increase the value of C the negative region will
expand, and new negative regions will be formed round negatively
electrified bodies. For every negative region thus formed the
surrounding positive region acquires one degree of periphraxy.
As the different negative regions expand, two or more of them
may meet in a point or a line. If n + 1 negative regions meet,
the positive region loses n degrees of periphraxy, and the point
114.] THEIR NUMBER. 137
or the line in which they meet is a point or line of equilibrium
of the nth degree.
When C becomes equal to Fl the positive region is reduced to
the electrified point or conductor of highest potential, and has
therefore lost all its periphraxy. Hence, if each point or line of
equilibrium counts for one, two, or n according to its degree, the
number so made up by the points or lines now considered will
be one less than the number of negatively electrified bodies.
There are other points or lines of equilibrium which occur where
the positive regions become separated from each other, and the
negative region acquires periphraxy. The number of these, reck
oned according to their degrees, is one less than the number of
positively electrified bodies.
If we call a point or line of equilibrium positive when it is the
meeting-place of two or more positive regions, and negative when
the regions which unite there are negative, then, if there are p
bodies positively and n bodies negatively electrified, the sum of
the degrees of the positive points and lines of equilibrium will be
p — 1, and that of the negative ones « — 1.
But, besides this definite number of points and lines of equi
librium arising from the junction of different regions, there may
be others, of which we can only affirm that their number must be
even. For if, as the negative region expands, it meets itself, it
becomes a cyclic region, and it may acquire, by repeatedly meeting
itself, any number of degrees of cyclosis, each of which corresponds
to the point or line of equilibrium at which the cyclosis was
established. As the negative region continues to expand till it
fills all space, it loses every degree of cyclosis it has acquired, and
becomes at last acyclic. Hence there is a set of points or lines
of equilibrium at which cyclosis is lost, and these are equal in
number of degrees to those at which it is acquired. .
If the form of the electrified bodies or conductors is arbitrary,
we can only assert that the number of these additional points or
lines is even, but if they are electrified points or spherical con
ductors, the number arising in this way cannot exceed (n— l)(n — 2),
where n is the number of bodies.
114.] The potential close to any point P may be expanded in
the series
F= ro+^ + tfa + fcc.;
where Hlt H2> &c. are homogeneous functions of #, y, z, whose
dimensions are 1, 2, &c. respectively.
138 POINTS AND LINES OF EQUILIBRIUM. [H5-
Since the first derivatives of V vanish at a point of equilibrium,
H± = 0, if P be a point of equilibrium.
Let Hi be the first function which does not vanish, then close to
the point P we may neglect all functions of higher degrees as
compared with Hi.
Now Hi — 0
is the equation of a cone of the degree i, and this cone is the cone
of closest contact with the equipotential surface at P.
It appears, therefore, that the equipotential surface passing
through P has, at that point, a conical point touched by a cone
of the second or of a higher degree.
If the point P is not on a line of equilibrium this cone
does not intersect itself, but consists of i sheets or some smaller
number.
If the nodal line intersects itself, then the point P is on a line
of equilibrium, and the equipotential surface through P cuts itself
in that line.
If there are intersections of the nodal line not on opposite points
of the sphere, then P is at the intersection of three or more lines
of equilibrium. For the equipotential surface through P must cut
itself in each line of equilibrium.
115.] If two sheets of the same equipotential surface intersect,
they must intersect at right angles.
For let the tangent to the line of intersection be taken as the
axis of 0, then -jy = 0. Also let the axis of x be a tangent to
d27
one of the sheets, then -=-g = 0. It follows from this, by Laplace's
dPV
equation, that -j-^ = 0, or the axis of y is a tangent to the other
y
sheet.
This investigation assumes that H2 is finite. If H2 vanishes, let
the tangent to the line of intersection be taken as the axis of z, and
lei as = r cos 0, and y = r sin 6, then, since
d27
dz2 " °' dx
or
dr* ^ r dr ^ r<
the solution of which equation in ascending powers of r is
1 1 6.] THEIR PROPERTIES. 139
At a point of equilibrium Al is zero. If the first term that does
not vanish is that in r*, then
V— F0 = ^r'cos (£0 + 0^) + terms in higher powers of r.
This gives i sheets of the equipotential surface F= F0, intersecting
at angles each equal to - . This theorem was given by Rankine *.
It is only under certain conditions that a line of equilibrium can
exist in free space, but there must be a line of equilibrium on the
surface of a conductor whenever the electrification of the conductor
is positive in one portion and negative in another.
In order that a conductor may be oppositely electrified in different
portions of its surface, there must be in the field some places where
the potential is higher than that of the body and others where it is
lower. We must remember that at an infinite distance the potential
is zero.
Let us begin with two conductors electrified positively to the
same potential. There will be a point of equilibrium between the
two bodies. Let the potential of the first body be gradually raised.
The point of equilibrium will approach the other body, and as the
process goes on it will coincide with a point on its surface. If the
potential of the first body be now increased, the equipotential
surface round the first body which has the same potential as the
second body will cut the surface of the second body at right angles
in a closed curve, which is a line of equilibrium.
Earnshaw's Theorem. ^
116.] An electrified body placed in a field of electric force cannot
be in stable equilibrium.
First, let us suppose the electricity of the moveable body (A), and
also that of the system of surrounding bodies (£), to be fixed in
those bodies.
Let V be the potential at any point of the moveable body due to
the action of the surrounding bodies (B)> and let e be the electricity
on a small portion of the moveable body A surrounding this point.
Then the potential energy of A with respect to B will be
M= 2(F*),
where the summation is to be extended to every electrified portion
of ^.
* ' Summary of the Properties of certain Stream Lines,' Phil. Mag., Oct. 1864.
See also, Thomson and Tait's Natural Philosophy, § 780 ; and Rankine ami Stokes,
in the Proc. R. S., 1867, p. 468 ; also W. R. Smith, Proc. R. S. Edin., 1869-70, p. 79.
140 POINTS AND LINES OF EQUILIBRIUM.
Let a, bj c be the coordinates of any electrified part of A with
respect to axes fixed in A, and parallel to those of x, y, z. Let the
coordinates of the point fixed in the body through which these axes
pass be £ 77, f
Let us suppose for the present that the body A is constrained to
move parallel to itself, then the absolute coordinates of the point
a, b3 c will be
x — f+«, y — y + b, z — f+ c.
The potential of the body A with respect to B may now be
expressed as the sum of a number of terms, in each of which V
is expressed in terms of <z, b} c and f, 17, £ and the sum of these
terms is a function of the quantities #, b, c, which are constant for
each point of the body, and of £ 77, f, which vary when the body is
moved.
Since Laplace's equation is satisfied by each of these terms it is
satisfied by their sum, or
" Jf " ~W ''
Now let a small displacement be given to A, so that
d£ = ldrt dr] = mdr, d£ = ndr;
then -=— dr will be the increment of the potential of A with respect
to the surrounding system B.
If this be positive, work will have to be done to increase r, and
there will be a force -=— tending to diminish r and to restore A to
its former position, and for this displacement therefore the equi
librium will be stable. If, on the other hand, this quantity is
negative, the force will tend to increase r, and the equilibrium will
be unstable.
Now consider a sphere whose centre is the origin and whose
radius is r, and so small that when the point fixed in the body
lies within this sphere no part of the moveable body A can coincide
with any part of the external system B. Then, since within the
sphere \?2M ' = 0, the surface-integral
-i-dS= 0,
dr
taken over the surface of the sphere.
Hence, if at any part of the surface of the sphere —7— is positive,
CIT
there must be some other part of the surface where it is negative,
Il6.] EQUILIBRIUM ALWAYS UNSTABLE. 141
and if the body A be displaced in a direction in which —=- is
negative, it will tend to move from its original position, and its
equilibrium is therefore necessarily unstable.
The body therefore is unstable even when constrained to move
parallel to itself, a fortiori it is unstable when altogether free.
Now let us suppose that the body A is a conductor. We might
treat this as a case of equilibrium of a system of bodies, the move-
able electricity being considered as part of that system, and we
might argue that as the system is unstable when deprived of so
many degrees of freedom by the fixture of its electricity, it must
a fortiori be unstable when this freedom is restored to it.
But we may consider this case in a more particular way, thus —
First, let the electricity be fixed in A, and let it move through
the small distance dr. The increment of the potential of A due to
... . dM ,
this cause is — r— dr.
dr
Next, let the electricity be allowed to move within A into its
position of equilibrium, which is always stable. During this motion
the potential will necessarily be diminished by a quantity which we
may call Gdr.
Hence the total increment of the potential when the electricity
is free to move will be
fdM /A*
(-dr--C^r>
and the force tending to bring A back towards its original position
will be dM
~dr~"Cj
where C is always positive.
Now we have shewn that — v— is neg-ative for certain direc-
dr
tions of r, hence when the electricity is free to move the instability
in these directions will be increased.
CHAPTER VII.
FORMS OF THE EQUIPOTENTIAL SURFACES AND LINES OF
INDUCTION IN SIMPLE CASES.
117.] WE have seen that the determination of the distribution
of electricity on the surface of conductors may be made to depend
on the solution of Laplace's equation
^ _
~d^ + df + dz* = '
V being a function of x, y^ and z, which is always finite and con
tinuous, which vanishes at an infinite distance, and which has
a given constant value at the surface of each conductor.
It is not in general possible by known mathematical methods
to solve this equation so as to fulfil arbitrarily given conditions,
but it is always possible to assign various forms to the function
V which shall satisfy the equation, and to determine in each case
the forms of the conducting surfaces, so that the function V shall
be the true solution.
It appears, therefore, that what we should naturally call the
inverse problem of determining the forms of the conductors from
the potential is more manageable than the direct problem of de
termining the potential when the form of the conductors is given.
In fact, every electrical problem of which we know the solution
has been constructed by an inverse process. It is therefore of
great importance to the electrician that he should know what
results have been obtained in this way, since the only method by
which he can expect to solve a new problem is by reducing it
to one of the cases in which a similar problem has been con
structed by the inverse process.
This historical knowledge of results can be turned to account in
two ways. If we are required to devise an instrument for making
electrical measurements with the greatest accuracy, we may select
those forms for the electrified surfaces which correspond to cases
of which we know the accurate solution. If, on the other hand,
Jl8.] USE OF DIAGRAMS. 143
we are required to estimate what will be the electrification of bodies
whose forms are given, we may begin with some case in which one
of the equipotential surfaces takes a form somewhat resembling the
given form,, and then by a tentative method we may modify the pro
blem till it more nearly corresponds to the given case. This method
is evidently very imperfect considered from a mathematical point
of view, but it is the only one we have, and if we are not allowed
to choose our conditions, we can make only an approximate cal
culation of the electrification. It appears, therefore, that what we
want is a knowledge of the forms of equipotential surfaces and
lines of induction in as many different cases as we can collect
together and remember. In certain classes of cases, such as those
relating to spheres, we may proceed by mathematical methods. In
other cases we cannot afford to despise the humbler method of
actually drawing tentative figures on paper, and selecting that
which appears least unlike the figure we require.
This latter method I think may be of some use, even in cases in
which the exact solution has been obtained, for I find that an eye-
knowledge of the forms of the equipotential surfaces often leads to a
right selection of a mathematical method of solution.
I have therefore drawn several diagrams of systems of equipotential
surfaces and lines of force, so that the student may make himself
familiar with the forms of the lines. The methods by which such
diagrams may be drawn will be explained as we go on, as they
belong to questions of different kinds.
118.] In the first figure at the end of this volume we have the
equipotential surfaces surrounding two points electrified with quan
tities of electricity of the same kind and in the ratio of 20 to 5.
Here each point is surrounded by a system of equipotential
surfaces which become more nearly spheres as they become smaller,
but none of them are accurately spheres. If two of these surfaces,
one surrounding each sphere, be taken to represent the surfaces
of two conducting bodies, nearly but not quite spherical, and if
these bodies be charged with the same kind of electricity, the
charges being as 4 to 1, then the diagram will represent the
equipotential surfaces, provided we expunge all those which are
drawn inside the two bodies. It appears from the diagram that
the action between the bodies will be the same as that between
two points having the same charges, these points being not exactly
in the middle of the axis of each body, but somewhat more remote
than the middle point from the other body.
144 EQUIPOTENTIAL SURFACES \_119'
The same diagram enables us to see what will be the distribution
of electricity on one of the oval figures, larger at one end than
the other, which surround both centres. Such a body, if electrified
with a charge 25 and free from external influence, will have the
surface-density greatest at the small end, less at the large end,
and least in a circle somewhat nearer the smaller than the larger end.
There is one equipotential surface, indicated by a dotted line,
which consists of two lobes meeting at the conical point P. That
point is a point of equilibrium, and the surface-density on a body
of the form of this surface would be zero at this point.
The lines of force in this case form two distinct systems, divided
from one another by a surface of the sixth degree, indicated by a
dotted line, passing through the point of equilibrium, and some
what resembling one sheet of the hyperboloid of two sheets.
This diagram may also be taken to represent the lines of force
and equipotential surfaces belonging to two spheres of gravitating
matter whose masses are as 4 to 1.
119.] In the second figure we have again two points whose
charges are as 4 to 1, but the one positive and the other negative.
In this case one of the equipotential surfaces, that, namely, corre
sponding to potential zero, is a sphere. It is marked in the diagram
by the dotted circle Q. The importance of this spherical surface
will be seen when we come to the theory of Electrical Images.
We may see from this diagram that if two round bodies are
charged with opposite kinds of electricity they will attract each other
as much as two points having the same charges but placed some
what nearer together than the middle points of the round bodies.
Here, again, one of the equipotential surfaces, indicated by a
dotted line, has two lobes, an inner one surrounding the point whose
charge is 5 and an outer one surrounding both bodies, the two
lobes meeting in a conical point P which is a point of equilibrium.
If the surface of a conductor is of the form of the outer lobe, a
roundish body having, like an apple, a conical dimple at one end of
its axis, then, if this conductor be electrified, we shall be able to
determine the superficial density at any point. That at the bottom
of the dimple will be zero.
Surrounding this surface we have others having a rounded
dimple which flattens and finally disappears in the equipotential
surface passing through the point marked M.
The lines of force in this diagram form two systems divided by a
surface which passes through the point of equilibrium.
121.] AND LINES OF INDUCTION. 145
If we consider points on the axis on the further side of the point
J5, we find that the resultant force diminishes to the double point P,
where it vanishes. It then changes sign, and reaches a maximum
at M, after which it continually diminishes.
This maximum, however,, is only a maximum relatively to other
points on the axis, for if we draw a surface perpendicular to the
axis, H is a point of minimum force relatively to neighbouring
points on that surface.
120.] Figure III represents the equipotential surfaces and lines
of force due to an electrified point whose charge is 10 placed at
A, and surrounded by a field of force, which, before the intro
duction of the electrified point, was uniform in direction and
magnitude at every part. In this case, those lines of force which
belong to A are contained within a surface of revolution which
has an asymptotic cylinder, having its axis parallel to the un
disturbed lines of force.
The equipotential surfaces have each of them an asymptotic
plane. One of them, indicated by a dotted line, has a conical
point and a lobe surrounding the point A. Those below this surface
have one sheet with a depression near the axis. Those above have
a closed portion surrounding A and a separate sheet with a slight
depression near the axis.
If we take one of the surfaces below A as the surface of a con
ductor, and another a long way below A as the surface of another
conductor at a different potential, the system of lines and surfaces
between the two conductors will indicate the distribution of electric
force. If the lower conductor is very far from A its surface will
be very nearly plane, so that we have here the solution of the
distribution of electricity on two surfaces, both of them nearly
plane and parallel to each other, except that the upper one has
a protuberance near its middle point, which is more or less pro
minent according to the particular equipotential line we choose for
the surface.
121.] Figure IV represents the equipotential surfaces and lines
of force due to three electrified points A} B and C, the charge of A
being 1 5 units of positive electricity, that of B 1 2 units of negative
electricity, and that of C 20 units of positive electricity. These
points are placed in one straight line, so that
AB = 9, BC = 16, AC = 25.
In this case, the surface for which the potential is unity consists
of two spheres whose centres are A and C and their radii 15 and 20.
VOL. i. L
146 EQUIPOTENTIAL SURFACES
These spheres intersect in the circle which cuts the plane of the
paper in I) and I/, so that B is the centre of this circle and its
radius is 12. This circle is an example of a line of equilibrium, for
the resultant force vanishes at every point of this line.
If we suppose the sphere whose centre is A to be a conductor
with a charge of 3 units of positive electricity, and placed under
the influence of 20 units of positive electricity at C, the state of
the case will be represented by the diagram if we leave out all the
lines within the sphere A. The part of this spherical surface within
the small circle DD' will be negatively electrified by the influence
of C. All the rest of the sphere will be positively electrified,, and
the small circle DD' itself will be a line of no electrification.
We may also consider the diagram to represent the electrification
of the sphere whose centre is C, charged with 8 units of positive
electricity, and influenced by 1 5 units of positive electricity placed
at A.
The diagram may also be taken to represent the case of a con
ductor whose surface consists of the larger segments of the two
spheres meeting in Lit, charged with 23 units of positive elec
tricity.
We shall return to the consideration of this diagram as an
o
illustration of Thomson's Theory of Electrical Images. See Art. 168.
122.] I am anxious that these diagrams should be studied as
'illustrations of the language of Faraday in speaking of ' lines of
force/ the ' forces of an electrified body,' &c.
In strict mathematical language the word Force is used to signify
the supposed cause of the tendency which a material body is found
to have towards alteration in its state of rest or motion. It is
indifferent whether we speak of this observed tendency or of its
immediate cause, since the cause is simply inferred from the effect,
and has no other evidence to support it.
Since, however, we are ourselves in the practice of directing the
motion of our own bodies, and of moving other things in this way,
we have acquired a copious store of remembered sensations relating
to these actions, and therefore our ideas of force are connected in
our minds with ideas of conscious power, of exertion, and of fatigue,
and of overcoming or yielding to pressure. These ideas, which give
a colouring and vividness to the purely abstract idea of force, do
not in mathematically trained minds lead to any practical error.
But in the vulgar language of the time when dynamical science
was unknown, all the words relating to exertion, such as force,
123.] AND ^INES OF INDUCTION. 147
energy, power, &c., were confounded with each other, though some
of the schoolmen endeavoured to introduce a greater precision into
their language.
The cultivation and popularization of correct dynamical ideas
since the time of Galileo and Newton has effected an immense
change in the language and ideas of common life, but it is only
within recent times, and in consequence of the increasing im
portance of machinery, that the ideas of force, energy, and power
have become accurately distinguished from each other. Very few,
however, even of scientific men, are careful to observe these dis
tinctions ; hence we often hear of the force of a cannon-ball when
either its energy or its momentum is meant, and of the force of an
electrified body when the quantity of its electrification is meant.
Now the quantity of electricity in a body is measured, according
to Faraday's ideas, by the number of lines of force, or rather of
induction, which proceed from it. These lines of force must all
terminate somewhere, either on bodies in the neighbourhood, or on
the walls and roof of the room, or on the earth, or on the heavenly
bodies, and wherever they terminate there is a quantity of elec
tricity exactly equal and opposite to that on the part of the body
from which they proceeded. By examining the diagrams this will
be seen to be the case. There is therefore no contradiction between
Faraday's views and the mathematical results of the old theor}^,
but, on the contrary, the idea of lines of force throws great light
on these results, and seems to afford the means of rising by a con
tinuous process from the somewhat rigid conceptions of the old
theory to notions which may be capable of greater expansion, so
as to provide room for the increase of our knowledge by further
researches.
123.] These diagrams are constructed in the following manner : —
First, take the case of a single centre of force, a small electrified
-p
body with a charge E. The potential at a distance r is F = — ;
T?
hence, if we make r= -^ . we shall find r, the radius of the sphere
for which the potential is F. If we now give to F the values
1, 2, 3, &c., and draw the corresponding spheres, we shall obtain
a series of equipotential surfaces, the potentials corresponding to
which are measured by the natural numbers. The sections of these
spheres by a plane passing through their common centre will be
circles, which we may mark with the number denoting the potential
L 2
148 EQUIPOTENTIAL SURFACES [I23-
of each. These are indicated by the dotted circles on the right
hand of Fig-. 6.
If there be another centre of force, we may in the same way draw
the equipotential surfaces belonging to it, and if we now wish to
find the form of the equipotential surfaces due to both centres
together, we must remember that if V^ be the potential due to one
centre, and V2 that due to the other, the potential due to both will be
V^ + V^ = V. Hence, since at every intersection of the equipotential
surfaces belonging to the two series we know both V^ and F2, we
also know the value of V. If therefore we draw a surface which
passes through all those intersections for which the value of V is
the same, this surface will coincide with a true equipotential surface
at all these intersections, and if the original systems of surfaces
be drawn sufficiently close, the new surface may be drawn with
any required degree of accuracy. The equipotential surfaces due to
two points whose charges are equal and opposite are represented by
the continuous lines on the right hand side of Fig. 6.
This method may be applied to the drawing of any system of
equipotential surfaces when the potential is the sum of two po
tentials, for which we have already drawn the equipotential surfaces.
The lines of force due to a single centre of force are straight
lines radiating from that centre. If we wish to indicate by these
lines the intensity as well as the direction of the force at any point,
we must draw them so that they mark out on the equipotential
surfaces portions over which the surface-integral of induction has
definite values. The best way of doing this is to suppose our
plane figure to be the section of a figure in space formed by the
revolution of the plane figure about an axis passing through the
centre of force. Any straight line radiating from the centre and
making an angle 6 with the axis will then trace out a cone,
and the surface-integral of the induction through that part of any
surface which is cut off by this cone on the side next the positive
direction of the axis, is 2ir^E(l— cos 6).
If we further suppose this surface to be bounded by its inter
section with two planes passing through the axis, and inclined at
the angle whose arc is equal to half the radius, then the induction
through the surface so bounded is
E (1 —cos 0) = 2 V, say ;
and 0 = cos"1 (l — 2 -^f)-
If we now give to ^ a series of values 1, 2, 3 ... E, we shall find
VoC. 2.
1C face, f 148.
Fig. 6.
lanes of Ihrce.
l' Surfaces
Jfet/uxl* of
Zirtes of Forre <??ta
urface.
CZa render: .
123.] AND LINES OF INDUCTION. 149
a corresponding series of values of 9, and if E be an integer, the
number of corresponding lines of force, including the axis, will be
equal to E.
We have therefore a method of drawing lines of force so that
the charge of any centre is indicated by the number of lines which
converge to it, and the induction through any surface cut off in the
way described is measured by the number of lines of force which
pass through it. The dotted straight lines on the left hand side
of Fig. 6 represent the lines of force due to each of two electrified
points whose charges are 10 and —10 respectively.
If there are two centres of force on the axis of the figure we
may draw the lines of force for each axis corresponding to values
of ^ and ^2, and then, by drawing lines through the consecutive
intersections of these lines, for which the value of ^ + ^2 is the
same, we may find the lines of force due to both centres, and in
the same way we may combine any two systems of lines of force
which are symmetrically situated about the same axis. The con
tinuous curves on the left hand side of Fig. 6 represent the lines
of force due to the tsvo electrified points acting at once.
After the equipotential surfaces and lines of force have been
constructed by this method the accuracy of the drawing may be
tested by observing whether the two systems of lines are every
where orthogonal, and whether the distance between consecutive
eqiipotential surfaces is to the distance between consecutive lines
of force as half the distance from the axis is to the assumed unit of
length.
In the case of any such system of finite dimensions the line of
force whose index number is ^ has an asymptote which passes
through the centre of gravity of the system, and is inclined to the
•^/
axis at an angle whose cosine is 1 — 2 -^ , where E is the total
electrification of the system, provided ^ is less than E. Lines of
force whose index is greater than E are finite lines.
The lines of force corresponding to a field of uniform force parallel
to the axis are lines parallel to the axis, the distances from the
axis being the square roots of an arithmetical series.
The theory of equipotential surfaces and lines of force in two
dimensions will be given when we come to the theory of conjugate
functions *.
* See a paper ' On the Flow of Electricity in Conducting Surfaces,' by Prof. W. R.
Smith, Proc. R. S. Edin., 1869-70, p. 79.
CHAPTER VIII.
SIMPLE CASES OP ELECTEIFICATIOtf.
Two Parallel Planes.
124.] We shall consider, in the first place, two parallel plane
conducting surfaces of infinite extent, at a distance c from each
other, maintained respectively at potentials A and B.
It is manifest that in this case the potential V will be a function
of the distance z from the plane A, and will be the same for all
points of any parallel plane between A and J3, except near the
boundaries of the electrified surfaces, which by the supposition
are at an infinitely great distance from the point considered.
Hence, Laplace's equation becomes reduced to
__
== '
the integral of which is
7= C, + C2z;
and since when z = 0, V = A, and when z = <?, V = B,
For all points between the planes, the resultant electrical force
is normal to the planes, and its magnitude is
c
In the substance of the conductors themselves, R = 0. Hence
the distribution of electricity on the first plane has a surface-
density <r, where A—B
47TO- = R = --
c
On the other surface, where the potential is jB, the surface-
density a-' will be equal and opposite to <r, and
1 24.] SIMPLE CASES. PARALLEL PLANES. 151
Let us next consider a portion of the first surface whose area
is S, taken so that no part of S is near the boundary of the
surface.
The quantity of electricity on this surface is E± = S<r, and, by
Art. 79, the force acting on every unit of electricity is \R, so that
the whole force acting on the area S, and attracting it towards
the other plane, is
Here the attraction is expressed in terms of the area S, the
difference of potentials of the two surfaces (A — B), and the distance
between them c. The attraction, expressed in terms of the charge
E} on the area S, is 2 TT
The electrical energy due to the distribution of electricity on the
area S, and that on an area S' on the surface B denned by projecting
S on the surface B by a system of lines of force, which in this case
are normals to the planes, is
Q=
-
2
-27r E*c
- -3- A c,
= Fc.
The first of these expressions is the general expression of elec
trical energy.
The second gives the energy in terms of the area, the distance,
and the difference of potentials.
The third gives it in terms of the resultant force R, and the
volume Sc included between the areas S and S', and shews that the
energy in unit of volume isp where 8 nfl = R2.
The attraction between the planes is jo/S> or in other words, there
is an electrical tension (or negative pressure) equal to p on every
unit of area.
The fourth expression gives the energy in terms of the charge.
The fifth shews that the electrical energy is equal to the work
which would be done by the electric force if the two surfaces were
to be brought together, moving parallel to themselves, with their
electric charges constant.
152 SIMPLE CASES.
To express the charge in terms of the difference of potentials,
we have i
1 o
The coefficient — — = q represents the charge due to a differ
ence of potentials equal to unity. This coefficient is called the
Capacity of the surface S, due to its position relatively to the
opposite surface.
Let us now suppose that the medium between the two surfaces
is no longer air but some other dielectric substance whose specific
inductive capacity is K, then the charge due to a given difference
of potentials will be K times as great as when the dielectric is air,
or
The total energy will be
_ 2^
- gjgJSl C.
The force between the surfaces will be
_ KS (B-A)*
--E*
~ KS l '
Hence the force between two surfaces kept at given potentials
varies directly as K, the specific capacity of the dielectric, but the
force between two surfaces charged with given quantities of elec
tricity varies inversely as K.
Two Concentric Spherical Surfaces.
125.] Let two concentric spherical surfaces of radii a and £, of
which I is the greater, be maintained at potentials A and B
respectively, then it is manifest that the potential V is a function
of r the distance from the centre. In this case, Laplace's equation
becomes d*V 2 dV
~W + r ~dr = °'
The integral of this is
F=Q+Qr-i;
and the condition that V— A when r = a, and V = B when r = 6,
gives for the space between the spherical surfaces,
12 5-] CONCENTRIC SPHERICAL SURFACES. 153
Aa-Bb A-B
r=
d — 1) & i — I)
dV A-B
_2
If (715 <r2 are the surface-densities on the opposed surfaces of a
solid sphere of radius a, and a spherical hollow of radius b, then
1 A-B 1 B-A
If EI and ^2 be the whole charges of electricity on these surfaces,
Tlie capacity of the enclosed sphere is therefore 7—
If the outer surface of the shell be also spherical and of radius c,
then, if there are no other conductors in the neighbourhood, the
charge on the outer surface is
E3 = Be.
Hence the whole charge on the inner sphere is
and that of the outer
If we put £ = oo, we have the case of a sphere in an infinite
space. The electric capacity of such a sphere is a, or it is nu
merically equal to its radius.
The electric tension on the inner sphere per unit of area is
(A -By2
STT a2 (b-a)2
The resultant of this tension over a hemisphere is ira2j) = F
normal to the base of the hemisphere, and if this is balanced by a
surface tension exerted across the circular boundary of the hemi
sphere, the tension on unit of length being T, we have
F= 2iraT.
b* (A-B)2 Ef
Hence
l -
8 (b — of 8 a
16iro (b-a
154 SIMPLE CASES. [126.
If a spherical soap bubble is electrified to a potential A, then, if
its radius is a, the charge will be Aa, and the surface-density
will be I A
47T a
The resultant electrical force just outside the surface will be 4770-,
and inside the bubble it is zero, so that by Art. 79 the electrical
force on unit of area of the surface will be 27ro-2, acting outwards.
Hence the electrification will diminish the pressure of the air
within the bubble by 27ro-2, or
But it may be shewn that if T is the tension which the liquid
film exerts across a line of unit length, then the pressure from
T
within required to keep the bubble from collapsing is 2 - . If the
electrical force is just sufficient to keep the bubble in equilibrium
when the air within and without is at the same pressure
A2 = IGvaT.
Two Infinite Coaxal Cylindric Surfaces.
126.] Let the radius of the outer surface of a conducting cylinder
be «, and let the radius of the inner surface of a hollow cylinder,
having the same axis with the first, be I. Let their potentials
be A and B respectively. Then, since the potential V is in this
case a function of r, the distance from the axis, Laplace's equation
becomes
d2F \_dV_
dr2 +r~fo == °'
whence V = Q + C2 log r.
Since V = A when r = a, and V = B when r = b,
A log — |- .Slog -
V = r a -
If o-j, o-2 are the surface-densities on the inner and outer
surfaces,
A-B B-A
47701 = - -, 4770*2 =
I2/.] COAXAL CYLINDERS. 155
If El and E2 are the charges on a portion of the two cylinders of
length I, measured along the axis,
^A — B
The capacity of a length I of the interior cylinder is therefore
!**£
If the space between the cylinders is occupied by a dielectric of
specific capacity K instead of air, then the capacity of the inner
cylinder is L IK
2 £-'
The energy of the electrical distribution on the part of the infinite
cylinder which we have considered is
lK(A—Bf
4 , b
1
Ir»
i
i
Fig. 5.
127.] Let there be two hollow cylindric conductors A and B,
Fig. 5, of indefinite length,, having the axis of x for their common
axis, one on the positive and the other on the negative side of the
origin, and separated by a short interval near the origin of co
ordinates.
Let a hollow cylinder C of length 2 1 be placed with its middle
point at a distance x on the positive side of the origin, so as to
extend into both the hollow cylinders.
Let the potential of the positive hollow cylinder be A, that of
the negative one J3, and that of the internal one C, and let us put
a for the capacity per unit of length of C with respect to A, and
/3 for the same quantity with respect to B.
The capacities of the parts of the cylinders near the origin and
near the ends of the inner cylinder will not be affected by the
value of x provided a considerable length of the inner cylinder
enters each of the hollow cylinders. Near the ends of the hollow
156 SIMPLE CASES. [127.
cylinders, and near the ends of the inner cylinder, there will be
distributions of electricity which we are not yet able to calculate,
but the distribution near the origin will not be altered by the
motion of the inner cylinder provided neither of its ends comes
near the origin, and the distributions at the ends of the inner
cylinder will move with it, so that the only effect of the motion
will be to increase or diminish the length of those parts of the
inner cylinder where the distribution is similar to that on an in
finite cylinder.
Hence the whole energy of the system will be, so far as it depends
on x,
Q= ±a(l + x)(C-A)2 + %(3 (l-x] (C-B)2 + quantities
independent of x ;
and the resultant force parallel to the axis of the cylinders will be
If the cylinders A and B are of equal section, a = (3} and
X = a(3-A)(C-lU + #)).
It appears, therefore, that there is a constant force acting on
the inner cylinder tending to draw it into that one of the outer
cylinders from which its potential differs most.
If C be numerically large and A + B comparatively small, then
the force is approximately x = a(B — A)C;
so that the difference of the potentials of the two cylinders can be
measured if we can measure X, and the delicacy of the measurement
will be increased by raising C, the potential of the inner cylinder.
This principle in a modified form is adopted in Thomson's
Quadrant Electrometer, Art. 219.
The same arrangement of three cylinders may be used as a
measure of capacity by connecting B and C. If the potential of
A is zero, and that of B and C is F, then the quantity of electricity
on A will be ^ = (ql3 + a (l+x)) V\
so that by moving C to the right till x becomes os+ f the capacity of
the cylinder £ becomes increased by the definite quantity af, where
.. .
a and b being the radii of the opposed cylindric surfaces.
CHAPTER IX.
SPHERICAL HARMONICS.
On Singular Points at which the Potential becomes Infinite.
128.] We have already shewn that the potential due to a
quantity of electricity e, condensed at a point whose coordinates
are (a, b, c\ is V---
where r is the distance from the point (a, 5, c) to the point (x, y, z),
and Y is the potential at the point (#, y, z].
At the point (a, b, c) the potential and all its derivatives hecome
infinite, hut at every other point they are finite and continuous,
and the second derivatives of V satisfy Laplace's equation.
Hence, the value of F, as given by equation (1), may be the
actual value of the potential in the space outside a closed surface
surrounding the point (a, b, c], but we cannot, except for purely
mathematical purposes, suppose this form of the function to hold
up to and at the point (#, b, c) itself. For the resultant force close
to the point would be infinite, a condition which would necessitate
a discharge through the dielectric surrounding the point, and
besides this it would require an infinite expenditure of work to
charge a point with a finite quantity of electricity.
We shall call a point of this kind an infinite point of degree zero.
The potential and all its derivatives at such a point are infinite,
but the product of the potential and the distance from the point
is ultimately a finite quantity e when the distance is diminished
without limit. This quantity e is called the charge of the infinite
point.
This may be shewn thus. If V be the potential due to other
electrified bodies, then near the point V is everywhere finite, and
the whole potential is
whence Vr = T'r+e.
158 SPHEEICAL HARMONICS.
When r is indefinitely diminished Tf remains finite, so that
ultimately yr _ €t
129.] There are other kinds of singular points, the properties of
which we shall now investigate, but, before doing so, we must define
some expressions which we shall find useful in emancipating our
ideas from the thraldom of systems of coordinates.
An axis is any definite direction in space. We may suppose
it defined in Cartesian coordinates by its three direction-cosines
I, m, n, or, better still, we may suppose a mark made on the surface
of a sphere where the radius drawn from the centre in the direction
of the axis meets the surface. We may call this point the Pole
of the axis. An axis has therefore one pole only, not two.
If through any point x. y, z a plane be drawn perpendicular to
the axis, the perpendicular from the origin on the plane is
p = Ix + my + nz. (2)
The operation d d d d
-jj- = l-=- +m— + »-=-» (3)
an, ax ay az
is called Differentiation with respect to an axis h whose direction-
cosines are I, m, n.
Different axes are distinguished by different suffixes.
The cosine of the angle between the vector r and any axis 7^
is denoted by A0 and the vector resolved in the direction of the
axis by^, where
A;/ = lifs + mty + niZ =&. (4)
The cosine of the angle between two axes ht and 7tj is denoted by
Hi where My = /, lj + % m, + «, », . (5)
From these definitions it is evident that
«•*-»•' V
Now let us suppose that the potential at the point (so, y, z) due
to a singular point of any degree placed at the origin is
If such a point be placed at the extremity of the axis h, the
potential at (x, y, z) will be
Mf((x-lk), (y-mh), (z-nh));
INFINITE POINTS. 159
and if a point in all respects equal and of opposite sign be placed
at the origin, the potential due to the pair of points will be
r=Mf{(x-lX), (y-mh), (z-nh)}-Mf(x,y, z\
7 •
= — Mh -^F(x, y, z) + terms containing h2.
If we now diminish h and increase M without limit, their product
Mh remaining constant and equal to M' ', the ultimate value of the
potential of the pair of points will be
V> -) satisfies Laplace's equation, then V, which is the
difference of two functions, each of which separately satisfies the
equation, must itself satisfy it.
If we begin with an infinite point of degree zero, for which
Fo = M0-> (10)
we shall get for a point of the first degree
A point of the first degree may be supposed to consist of two
points of degree zero, having equal and opposite charges MQ and
— iro, and placed at the extremities of the axis h. The length
of the axis is then supposed to diminish and the magnitude of the
charges to increase, so that their product M^k is always equal to
Jfj. The ultimate result of this process when the two points
coincide is a point of the first degree, whose moment is J/x and
whose axis is ^. A point of the first degree may therefore be
called a Double point.
By placing two equal and opposite points of the first degree at
the extremities of the second axis h.2, and making M^2 = M.2, we
get by the same process a point of the second degree whose potential
160 SPHERICAL HARMONICS.
We may call a point of the second degree a Quadruple point,
because it is constructed by making four points approach each
other. It has two axes, h^ and 7/2, and a moment M2. The di
rections of these two axes and the magnitude of the moment com
pletely define the nature of the point.
130.] Let us now consider an infinite point of degree i having
i axes, each of which is defined by a mark on a sphere or by two
angular coordinates, and having also its moment Mit so that it is
defined by 2^+1 independent quantities. Its potential is obtained
by differentiating F0 with respect to the i axes in succession, so
that it may be written
The result of the operation is of the form
where Yit which is called the Surface Harmonic, is a function of the
i cosines, Ax . . . A^ of the angles between r and the i axes, and of the
\i(i—\) cosines, j*12, &c. of the angles between the different axes
themselves. In what follows we shall suppose the moment Mi unity.
Every term of Yi consists of products of these cosines of the form
Ml2' ^34 ••• M2s-l'2s ^2s + l •• • \J
in which there are s cosines of angles between two axes, and i—2s
cosines of angles between the axes and the radius vector. As each
axis is introduced by one of the i processes of differentiation, the
symbol of that axis must occur once and only once among the
suffixes of these cosines.
Hence in every such product of cosines all the indices occur
once, and none is repeated.
The number of different products of s cosines with double suffixes,
and i — 2s cosines with single suffixes, is
N= - ~=r-5— (15)
-
For if we take any one of the N different terms we can form
from it 2s arrangements by altering the order of the suffixes of the
cosines with double suffixes. From any one of these, again, we
can form \s_ arrangements by altering the order of these cosines,
and from any one of these we can form ; i-2s arrangements by
altering the order of the cosines with single suffixes. Hence, with
out altering the value of the term we may write it in 28 s^ i-2s
130.] TRIGONOMETRICAL EXPRESSION. 161
different ways, and if we do so to all the terms, we shall obtain
the whole permutations of i symbols, the number of which is <j_.
Let the sum of all terms of this kind be written in th<T ab
breviated form vf\f-2« «>»
^ (^ M )•
If we wish to express that a particular symbol j occurs among
the A's only, or among the n's only, we write it as a suffix to the \
or the fji. Thus the equation
2 (A-2« M«) = 2 (A/-2- ft) + 2 (A*-2- M/) (16)
expresses that the whole system of terms may be divided into two
portions, in one of which the symbol/ occurs among the direction-
cosines of the radius vector, and in the other among the cosines
of the angles between the axes.
Let us now assume that up to a certain value of i
r, = 4,0 2 (A*) + ALl 2 (A-2 M!) + &c.
-Mt,82(A'-2V) + &c. (17)
This is evidently true when i — \ and when i = 2. We shall shew
that if it is true for i it is true for i + 1 . We may write the series
r;. = s{4,,2(v-'v)}, (is)
where S indicates a summation in which all values of s not greater
than \ i are to be taken.
Multiplying by _i_r~(i+l\ and remembering that p{ = r\i} we
obtain by (14), for the value of the solid harmonic of negative
degree, and moment unity,
V{= \^S{AitSr2s-2i-lI,(Iji-2s^}. (19)
Differentiating Vi with respect to a new axis whose svmbol is
y, we should obtain J^+1 with its sign reversed,
r2°-2i-1 2 (/-2'-V/+1)}- (20)
If we wish to obtain the terms containing s cosines with double
suffixes we must diminish s by unity in the second term, and we find
)]}. (21)
If we now make
1_s (22)
then Ti+l = i±lS {^+l_^2.-2(i+i)-i 2 ^+1-2.^ (23)
and this value of J^+1 is the same as that obtained by changing i
VOL. i.
162 SPHERICAL HARMONICS. [I3I«
into i+l in the assumed expression, equation (19), for V{. Hence
the assumed form of 7J", in equation (19), if true for any value of i,
is true for the next higher value.
To find the value of ALs, put s = 0 in equation (22), and we find
4+i.o = ^^ 4.o ; (24)
£-f- i
and therefore, since A1 0 is unity,
I2t
(25)
and from this we obtain, by equation (22), for the general value of
the coefficient 12«-2s
and finally, the value of the trigonometrical expression for Tt is
This is the most general expression for the spherical surface-
harmonic of degree i. If i points on a sphere are given, then, if any
other point P is taken on the sphere, the value of Yi for the point
P is a function of the i distances of P from the i points, and of the
\i(i— 1) distances of the i points from each other. These i points
may be called the Poles of the spherical harmonic. Each pole
may be defined by two angular coordinates, so that the spherical
harmonic of degree i has 2i independent constants, exclusive of its
moment, Mi9
131.] The theory of spherical harmonics* was first given by
Laplace in the third book of his Mecanique Celeste. The harmonics
themselves are therefore often called Laplace's Coefficients.
They have generally been expressed in terms of the ordinary
spherical coordinates 0 and 0, and contain 2i+l arbitrary con
stants. Gauss appears* to have had the idea of the harmonic
being determined by the position of its poles, but I have not met
with any development of this idea.
In numerical investigations I have often been perplexed on ac
count of the apparent want of definiteness of the idea of a Laplace's
Coefficient or spherical harmonic. By conceiving it as derived by
the successive differentiation of — with respect to i axes, and as
expressed in terms of the positions of its i poles on a sphere, I
* Gauss. Werlse, bd.v. s. 361.
132.] SYMMETRICAL SYSTEM. 163
have made the conception of the general spherical harmonic of any
integral degree perfectly definite to myself, and I hope also to those
who may have felt the vagueness of some other forms of the ex
pression.
When the poles are given, the value of the harmonic for a given
point on the sphere is a perfectly definite numerical quantity.
When the form of the function, however, is given, it is by no
means so easy to find the poles except for harmonics of the first
and second degrees and for particular cases of the higher degrees.
Hence, for many purposes it is desirable to express the harmonic
as the sum of a number of other harmonics, each of which has its
axes disposed in a symmetrical manner.
Symmetrical System.
132.] The particular forms of harmonics to which it is usual to
refer all others are deduced from the general harmonic by placing
i — (T of the poles at one point, which we shall call the Positive Pole
of the sphere, and the remaining a- poles at equal distances round
one half of the equator.
In this case A1? A2, ... A,-^ are each of them equal to cos 0, and
A.f-s+1 ... A^ are of the form sin 9 cos((£ — /3). We shall write /u for
cos 6 and v for sin 0.
Also the value of /*,-/ is unity if j and f are both less than i — cr,
zero when one is greater and the other less than this quantity,
and cos n - when both are greater.
When all the poles are concentrated at the pole of the sphere,
the harmonic becomes a zonal harmonic for which a- = 0. As the
zonal harmonic is of great importance we shall reserve for it the
symbol §»•
We may obtain its value either from the trigonometrical ex
pression (27), or more directly by differentiation, thus
n -n-
It is often convenient to express Qf as a homogeneous function of
cos 0 and sin 6, which we shall write //, and v respectively,
M 2
164 SPHERICAL HARMONICS. [X32-
(30)
In this expansion the coefficient of /^. is unity, and all the other
terms involve v. Hence at the pole, where ^=1 and v=0, Q{ = 1.
It is shewn in treatises on Laplace's Coefficients that Q{ is the
coefficient of Jil in the expansion of (1 — 2^/£ + ^2)~^.
The other harmonics of the symmetrical system are most con
veniently obtained by the use of the imaginary coordinates 'given by
Thomson and Tait, Natural Philosophy, vol. i. p. 148,
The operation of differentiating with respect to a axes in suc
cession, whose directions make angles — with each other in the
plane of the equator, may then be written
*1 = ^1+^1. (32)
The surface harmonic of degree i and type a is found by
differentiating - with respect to i axes, cr of which are at equal
intervals in the plane of the equator, while the remaining i — a
coincide with that of z, multiplying the result by ri+l and dividing
by _*_. Hence
'ro (£•+.£)&' (33)
Now £<T + ?]a' = 2 rcr2;0"cos(o-(^ + /3), (35)
and ^ ^ = (-1)-J=^ ^). (36)
Hence Y™ = 2 ^
where the factor 2 must be omitted when o- = 0.
The quantity 3 .°" ig a function of 0, the value of which is given
in Thomson and Tait's Natural Philosophy, vol. i. p. 149.
It may be derived from Q{ by the equation
_
where Qt- is expressed as a function of /x only.
1 33.] SOLID HARMONICS OF POSITIVE DEGREE. 165
Performing the differentiations on Q{ as given in equation (29),
we obtain
We may also express it as a homogeneous function of /* and y,
ir-- |i^^r -/1«-~«»,^{. (40)
22<r — — r
In this expression the coefficient of the first term is unity, and
the others may be written down in order by the application of
Laplace's equation.
The following relations will be found useful in Electrodynamics.
They may be deduced at once from the expansion of Q/i.
'«'-«" = = ' (41)
-1" 15" ~T'J''
0# &>&W Harmonics of Positive Degree.
133.] We have hitherto considered the spherical surface harmonic
Yi as derived from the solid harmonic
This solid harmonic is a homogeneous function of the coordinates
of the negative degree — (i+1). Its values vanish at an infinite
distance and become infinite at the origin.
We shall now shew that to every such function there corresponds
another which vanishes at the origin and has infinite values at an
infinite distance, and is the corresponding solid harmonic of positive
degree i.
A solid harmonic in general may be defined as a homogeneous
function of x, y^ and z, which satisfies Laplace's equation
d'2F d*7 d*V
~d^ + ~df + dz* ~~
Let Ht be a homogeneous function of the degree ^, such that
Ht = l^M^Yi = r2i+lF{. (43)
Then = 2i+lr2»-
166 SPHERICAL HARMONICS. [134-
Hence
,/ dV, dV: dV^ t
r^-l(x-^+y^ + z-^) + r 2^i_— l + --^ + — -i . 44)
> dx dy dz' V#2 dy- dz2J
t/ «7
Now, since Vi is a homogeneous function of negative degree i+1,
The first two terms therefore of the right hand member of
equation (44) destroy each other, and, since ^ satisfies Laplace's
equation, the third term is zero, so that Hi also satisfies Laplace's
equation, and is therefore a solid harmonic of degree i.
We shall next shew that the value of Hi thus derived from Vi is
of the most general form.
A homogeneous function of a?, y, z of degree i contains
i(t+i)(t+2)
terms. But
is a homogeneous function of degree ^ — 2, and therefore contains
\i(i— 1) terms, and the condition ^2HL = 0 requires that each of
these must vanish. There are therefore \i(i—l) equations between
the coefficients of the \ (i + 1)(^ + 2) terms of the homogeneous
function, leaving 2^+1 independent constants in the most general
form of H^
But we have seen that Jfi has 2^+1 independent constants,
therefore the value of Ht is of the most general form.
Application of Solid Harmonics to the Theory of Electrified Spheres.
134.] The function 7J satisfies the condition of vanishing at
infinity, but does not satisfy the condition of being everywhere
finite, for it becomes infinite at the origin.
The function IIi satisfies the condition of being finite and con
tinuous at finite distances from the origin, but does not satisfy the
condition of vanishing at an infinite distance.
But if we determine a closed surface from the equation
^=#0 (46)
and make Hi the potential function within the closed surface and
1 35.] ELECTRIFIED SPHERICAL SURFACE. 167
/^ the potential outside it, then by making- the surface-density a-
satisfy the characteristic equation
, (47)
we shall have a distribution of potential which satisfies all the
conditions.
It is manifest that if Hi and Vi are derived from the same value
of J"i5 the surface H{ = 1\ will be a spherical surface, and the
surface-density will also be derived from the same value of 1^.
Let a be the radius of the sphere, and let
(48)
Then at the surface of the sphere, where r = a,
dV dH
and -= --- =— = — 4770-;
dr dr
T)
or (j+i)__ + 2V-M = 477(7;
whence we find ffi and Jfi in terms of C,
We have now obtained an electrified system in which the potential
is everywhere finite and continuous. This system consists of a
spherical surface of radius a, electrified so that the surface-density
is everywhere CYit where C is some constant density and Yi is a
surface harmonic of degree i. The potential inside this sphere,
arising- from this electrification, is everywhere fft, and the potential
outside the sphere is T\.
These values of the potential within and without the sphere
might have been obtained in any given case by direct integration,
but the labour would have been great and the result applicable only
to the particular case.
135.] We shall next consider the action between a spherical
surface, rigidly electrified according to a spherical harmonic, and
an external electrified system which we shall call E.
Let V be the potential at any point due to the system E, and
Yi that due to the spherical surface whose surface-density is cr.
168 SPHERICAL HARMONICS. [_L35-
Then, by Green's theorem, the potential energy of E on the
electrified surface is equal to that of the electrified surface on E, or
(50)
where the first integration is to be extended over every element dS
of the surface of the sphere, and the summation 2 is to be extended
to every part dE of which the electrified system E is composed.
But the same potential function V{ may be produced by means
of a combination of 2* electrified points in the manner already
described. Let us therefore find the potential energy of E on
such a compound point.
If M0 is the charge of a single point of degree zero, then M0F
is the potential energy of V on that point.
If there are two such points, a positive and a negative one, at
the positive and negative ends of a line hlt then the potential energy
of E on the double point will be
and when M0 increases and &L diminishes indefinitely, but so that
1/0^ = Jl/i,
the value of the potential energy will be for a point of the first degree
Similarly for a point of degree i the potential energy with respect
to E will be
1
This is the value of the potential energy of E upon the singular
point of degree i. That of the singular point on E is ^dU3 and,
by Green's theorem, these are equal. Hence, by equation (50),
[[ &V
If o- = CTi where C is a constant quantity, then, by equations
(49) and (14),
. (51)
Hence, if V is any potential function whatever which satisfies
Laplace's equation within the spherical surface of radius a, then the
I37-] SURFACE-INTEGRAL OF THE PRODUCT OF HARMONICS. 169
integral of VYi dS, extended over every element dSt of the surface
of a sphere of radius a, is given by the equation
^•"-rfiirfar^s:' <52)
where the differentiations of V are taken with respect to the axes
of the harmonic Yit and the value of the differential coefficient is
that at the centre of the sphere.
136.] Let us now suppose that V is a solid harmonic of positive
degree j of the form j
T=^Y, (53)
At the spherical surface, r = a, the value of V is the surface har
monic YJ, and equation (52) becomes
II YY /<?
r<T>d =
where the value of the differential coefficient is that at the centre
of the sphere.
When / is numerically different from j, the surface-integral of
the product Yt Yj vanishes. For, when i is less than j, the result
of the differentiation in the second member of (54) is a homogeneous
function of x, y, and z, of degree j — i, the value of which at the
centre of the sphere is zero. If i is equal toj the result is a constant,
the value of which will be determined in the next article. If the
differentiation is carried further, the result is zero. Hence the
surface-integral vanishes when i is greater than j.
137.] The most important case is that in which the harmonic
rJYj is differentiated with respect to i new axes in succession, the
numerical value of J being the same as that of i, but the directions
of the axes being in general different. The final result in this case
is a constant quantity, each term being the product of i cosines of
angles between the different axes taken in pairs. The general
form of such a product may be written symbolically
which indicates that there are s cosines of angles between pairs of
axes of the first system and $ between axes of the second system,
the remaining i—2s cosines being between axes one of v.hich
belongs to the first and the other to the second system.
In each product the suffix of every one of the 2i axes occurs
once, and once only.
170 SPHEKICAL HARMONICS.
The number of different products for a given value of # is
([fji
N= ' (55)
The final result is easily obtained by the successive differen
tiation of
r,F. = —. , S {(_ 1V^=L r2.2 (y- V)} .
j j | j U t 2J-* j-s
Differentiating this i times in succession with respect to the new
axes, so as to obtain any given combination of the axes in pairs,
we find that in differentiating r2s with respect to s of the new axes,
which are to be combined with other axes of the new system, we
introduce the numerical factor 2s (2s — 2) ... 2, or 2s\s_. In con
tinuing the differentiation the j>/s become converted into /x's, but
no numerical factor is introduced. Hence
(56)
Substituting this result in equation (54) we find for the value of
the surface-integral of the product of two surface harmonics of the
same degree, taken over the surface of a sphere of radius a,
JJYiYidS =
This quantity differs from zero only when the two harmonics are
of the same degree, and even in this case, when the distribution of
the axes of the one system bears a certain relation to the distribution
of the axes of the other, this integral vanishes. In this case, the
two harmonics are said to be conjugate to each other.
On Conjugate Harmonics.
138.] If one harmonic is given, the condition that a second
harmonic of the same degree may be conjugate to it is expressed
by equating the right hand side of equation (57) to zero.
If a third harmonic is to be found conjugate to both of these
there will be two equations which must be satisfied by its 2i
variables.
If we go on constructing new harmonics, each of which is con
jugate to all the former harmonics, the variables will be continually
more and more restricted, till at last the (2i+ l)th harmonic will
have all its variables determined by the 2i equations, which must
1 3 9.] CONJUGATE HARMONICS. 171
be satisfied in order that it may be conjugate to the 2i preceding
harmonics.
Hence a system of 2i+l harmonics of degree i may be con
struct ed, each of which is conjugate to all the rest. Any other
harmonic of the same degree may be expressed as the sum of this
system of conjugate harmonics each multiplied by a coefficient.
The system described in Art. 132, consisting of 2^+1 har
monics symmetrical about a single axis, of which the first is zonal,
the next i— 1 pairs tesseral, and the last pair sectorial, is a par
ticular case of a system of 2i+l harmonics, all of which are
conjugate to each other. Sir W. Thomson has shewn how to
express the conditions that 2 i -f 1 perfectly general harmonics,
each of which, however, is expressed as a linear function of the
2 / -f 1 harmonics of this symmetrical system, may be conjugate
to each other. These conditions consist of i(2i+l) linear equa
tions connecting the (2^+l)2 coefficients which enter into the
expressions of the general harmonics in terms of the symmetrical
harmonics.
Professor Clifford has also shewn how to form a conjugate system
of 2»+l sectorial harmonics having different poles.
Both these results were communicated to the British Association
in 1871.
139.] If we take for Yj the zonal harmonic QJt we obtain a
remarkable form of equation (57).
In this case all the axes of the second system coincide with each
other.
The cosines of the form //,v will assume the form A. where A. is the
cosine of the angle between the common axis of Qj and an axis of
the first system.
The cosines of the form ^ will all become equal to unity.
The number of combinations of s symbols, each of which is
distinguished by two out of i suffixes, no suffix being repeated, is
N='' (58)
and when each combination is equal to unity this number represents
the sum of the products of the cosines p^, or 2 (/&,-/).
The number of permutations of the remaining I — 2s symbols of
the second set of axes taken all together is i-2s. Hence
2 fr/,./-2') = :«-2* 2 A'-2'. (59)
Equation (57) therefore becomes, when Yj is the zonal harmonic,
172 SPHERICAL HAKMONICS.
r,wl (so)
where J^-) denotes the value of Yi in equation (27) at the common
pole of all the axes of Qj.
140.] This result is a very important one in the theory of
spherical harmonics, as it leads to the determination of the form
of a series of spherical harmonics, which expresses a function having
any arbitrarily assigned value at each point of a spherical surface.
For let F be the value of the function at any given point of the
sphere, say at the centre of gravity of the element of surface dS,
and let Qt be the zonal harmonic of degree i whose pole is the point
P on the sphere, then the surface-integral
extended over the spherical surface will be a spherical harmonic
of degree i, because it is the sum of a number of zonal harmonics
whose poles are the various elements dS, each being multiplied by
FdS. Hence, if we make
we may expand F in the form
F= AJo + A^ + bc. + AiYi, (62)
or
1
471 a*
. (63)
This is the celebrated formula of Laplace for the expansion in
a series of spherical harmonics of any quantity distributed over
the surface of a sphere. In making use of it we are supposed to
take a certain point P on the sphere as the pole of the zonal
harmonic Q{, and to find the surface-integral
over the whole surface of the sphere. The result of this operation
when multiplied by 2i+I gives the value of AiYi at the point P.
and by making P travel over the surface of the sphere the value of
A{ Y{ at any other point may be found.
SPHERICAL HARMONIC ANALYSIS. 173
But A{¥i is a general surface harmonic of degree ?, and we wish
to break it up into the sum of a series of multiples of the 2e-f- 1
conjugate harmonics of that degree.
Let Pi be one of these conjugate harmonics of a particular type,
and let Bi % be the part of Ai Yi belonging to this type.
We must first find r r
(64)
which may be done by means of equation (57), making the second
set of poles the same, each to each, as the first set.
We may then find the coefficient Bi from the equation
* = -sffFP'*& (63)
For suppose F expanded in terms of spherical harmonics, and let
BjPj be any term of this expansion. Then, if the degree of Pj is
different from that of Pi3 or if, the degree being the same, Pj is
conjugate to Pi3 the result of the surface-integration is zero. Hence
the result of the surface-integration is to select the coefficient of the
harmonic of the same type as P{.
The most remarkable example of the actual development of a
function in a series of spherical harmonics is the calculation by
Gauss of the harmonics of the first four degrees in the expansion
of the magnetic potential of the earth, as deduced from observations
in various parts of the world.
He has determined the twenty-four coefficients of the three
conjugate harmonics of the first degree, the five of the second,
seven of the third, and nine of the fourth, all of the symmetrical
system. The method of calculation is given in his General Theory
of Terrestrial Magnetism.
141.] When the harmonic Pi belongs to the symmetrical system
we may determine the surface-integral of its square extended over
the sphere by the following method.
The value of i* Y? is, by equations (34) and (36),
and by equations (33) and (54),
Performing the difierentiations, we find that the only terms
which do not disappear are those which contain zi~<T. Hence
174 SPHERICAL HARMONICS. [H2-
(66)
except when o- = 0, in which case we have, by equation (GO),
These expressions give the value of the surface-integral of the
square of any surface harmonic of the symmetrical system.
We may deduce from this the value of the integral of the square
of the function 3>), given in Art. 132,
9 22°" i—a- (} v \2
n»)Va = " - _ - (l~> . (68)
Ul 2i+l \i + <r
This value is identical with that given by Thomson and Tait, and is
true without exception for the case in which a = 0.
142.] The spherical harmonics which I have described are those
of integral degrees. To enter on the consideration of harmonics
of fractional, irrational, or impossible degrees is beyond my purpose,
which is to give as clear an idea as I can of what these harmonics
are. I have done so by referring the harmonic, not to a system
of polar coordinates of latitude and longitude, or to Cartesian
coordinates, but to a number of points on the sphere, which I
have called the Poles of the harmonic. Whatever be the type
of a harmonic of the degree i, it is always mathematically possible
to find i points on the sphere which are its poles. The actual
calculation of the position of these poles would in general involve
the solution of a system of 2i equations of the degree i. The
conception of the general harmonic, with its poles placed in any
manner on the sphere^ is useful rather in fixing our ideas than in
making calculations. For the latter purpose it is more convenient
to consider the harmonic as the sum of 2i-\- 1 conjugate harmonics
of selected types, and the ordinary symmetrical system, in which
polar coordinates are used, is the most convenient. In this system
the first type is the zonal harmonic Q{, in which all the axes
coincide with the axis of polar coordinates. The second type is
that in which i — 1 of the poles of the harmonic coincide at the pole
of the sphere, and the remaining one is on the equator at the origin
of longitude. In the third type the remaining pole is at 90° of
longitude.
In the same way the type in which i — or poles coincide at the
pole of the sphere, and the remaining a are placed with their axes
1 43.] FIGURES OF SPHERICAL HARMONICS. 175
at equal intervals — round the equator, is the type 2 <r, if one of the
poles is at the origin of longitude, or the type 2 a- -f 1 if it is at
longitude — •
143.] It appears from equation (60) that it is always possible
to express a harmonic as the sum of a system of zonal harmonics
of the same degree, having their poles distributed over the surface
of the sphere. The simplification of this system, however, does not
appear easy. I have however, for the sake of exhibiting to the
eye some of the features of spherical harmonics, calculated the zonal
harmonics of the third and fourth degrees, and drawn, by the
method already described for the addition of functions, the equi-
potential lines on the sphere for harmonics which are the sums of
two zonal harmonics. See Figures VI to IX at the end of this
volume.
Fig. VI represents the sum of two zonal harmonics of the third
degree whose axes are inclined 120° in the plane of the paper, and
the sum is the harmonic of the second type in which a = 1 , the axis
being perpendicular to the paper.
In Fig. VII the harmonic is also of the third degree, but the
axes of the zonal harmonics of which it is the sum are inclined
90°, and the result is not of any type of the symmetrical system.
One of the nodal lines is a great circle, but the other two which are
intersected by it are not circles.
Fig. VIII represents the difference of two zonal harmonics of
the fourth degree whose axes are at right angles. The result is a
tesseral harmonic for which i = 4, a- = 2.
Fig. IX represents the sum of the same zonal harmonics. The
result gives some notion of one type of the more general har
monic of the fourth degree. In this type the nodal line on the
sphere consists of six ovals not intersecting each other. Within
these ovals the harmonic is positive, and in the sextuply connected
part of the spherical surface which lies outside the ovals, the har
monic is negative.
All these figures are orthogonal projections of the spherical
surface.
I have also drawn in Fig. V a plane section through the axis
of a sphere, to shew the equipotential surfaces and lines of force
due to a spherical surface electrified according to the values of a
spherical harmonic of the first degree.
176 SPHERICAL HARMONICS. [T44-
Within the sphere the equipotential surfaces are equidistant
planes, and the lines of force are straight lines parallel to the axis,
their distances from the axis being as the square roots of the
natural numbers. The lines outside the sphere may be taken as a
representation of those which would be due to the earth's magnetism
if it were distributed according to the most simple type.
144.] It appears from equation (52), by making i = 0, that if
V satisfies Laplace's equation throughout the space occupied by a
sphere of radius #, then the integral
(69)
where the integral is taken over the surface of the sphere, dS being
an element of that surface, and F0 is the value of V at the centre
of the sphere. This theorem may be thus expressed.
The value of the potential at the centre of a sphere is the mean
value of the potential for all points of its surface, provided the
potential be due to an electrified system, no part of which is within
the sphere.
It follows from this that if V satisfies Laplace's equation through
out a certain continuous region of space, and if, throughout a
finite portion, however small, of that space, Fis constant, it will
be constant throughout the whole continuous region.
If not, let the space throughout which the potential has a
constant value C be separated by a surface S from the rest of
the region in which its values differ from C, then it will always
be possible to find a finite portion of space touching S and out
side of it in which V is either everywhere greater or everywhere
less than C.
Now describe a sphere with its centre within S, and with part
of its surface outside S, but in a region throughout which the value
of V is every where greater or everywhere less than C.
Then the mean value of the potential over the surface of the
sphere will be greater than its value at the centre in the first case
and less in the second, and therefore Laplace's equation cannot
be satisfied throughout the space occupied by the sphere, contrary
to our hypothesis. It follows from this that if V^=C throughout
any portion of a connected region, V— C throughout the whole
of the region which can be reached in any way by a body 01
finite size without passing through electrified matter. (We sup
pose the body to be of finite size because a region in which V is
constant may be separated from another region in which it is
45-] THEOREM OF GAUSS. 177
variable by an electrified surface, certain points or lines of which
are not electrified, so that a mere point might pass out of the
region through one of these points or lines without passing
through electrified matter.) This remarkable theorem is due to
Gauss. See Thomson and Tait's Natural Philosophy^ § 497.
It may be shewn in the same way that if throughout any finite
portion of space the potential has a value which can be expressed
by a continuous mathematical formula satisfying Laplace's equation,
the potential will be expressed by the same formula throughout
every part of space which can be reached without passing through
electrified matter.
For if in any part of this space the value of the function is V ',
different from V, that given by the mathematical formula, then,
since both V and V satisfy Laplace's equation, U= V —V does.
But within a finite portion of the space [7=0, therefore by what
we have proved U = 0 throughout the whole space, or T'= V.
145.] Let Y{ be a spherical harmonic of i degrees and of any
type. Let any line be taken as the axis of the sphere, and let the
harmonic be turned into n positions round the axis, the angular
o
distance between consecutive positions being --
If we take the sum of the n harmonics thus formed the result
will be a harmonic of i degrees, which is a function of 6 and of the
sines and cosines of n$.
If_ n is less than i the result will be compounded of harmonics for
which s is zero or a multiple of n less than i, but if n is greater
than •/ the result is a zonal harmonic. Hence the following theorem :
Let any point be taken on the general harmonic Yit and let a
small circle be described with this point for centre and radius 0,
and let n points be taken at equal distances round this circle, then
if Q; is the value of the zonal harmonic for an angle 0, and if Y- is
the value of Yi at the centre of the circle, then the mean of the
n values of Yi round the circle is equal to Qt Y{ provided n is greater
than i.
If n is greater than i -f s, and if the value of the harmonic at
each point of the circle be multiplied by sin<S(£ or cos sty where
s is less than i, and the arithmetical mean of these products be
As, then if 3?*^ *s the value of £W for the angle 6, the coefficient
of sin sty or cos 8$ in the expansion of Yt will be
VOL. I. N
-178 SPHERICAL HARMONICS. [ 146.
In this way we may analyse Yi into its component conjugate
harmonics by means of a finite number of ascertained values at
selected points on the sphere.
Application of Spherical Harmonic Analysis to the Determination
of the Distribution of Electricity on Spherical and nearly Spherical
Conductors under the Action of known External Electrical Forces.
146.] We shall suppose that every part of the electrified system
which acts on the conductor is at a greater distance from the
centre of the conductor than the most distant part of the conductor
itself, or, if the conductor is spherical, than the radius of the
sphere.
Then the potential of the external system, at points within this
distance, may be expanded in a series of solid harmonics of positive
degree y = A^ + ^ r YI + &c + j . j. ^ (7 0)
The potential due to the conductor at points outside it may be
expanded in a series of solid harmonics of the same type, but of
negative degree
(71)
At the surface of the conductor the potential is constant and
equal, say, to C. Let us first suppose the conductor spherical and
of radius a. Then putting r = a, we have U+ ~F= C, or, equating
the coefficients of the different degrees,
3Q = a(C-AJ,
JB1=-a^A19 (72)
_#. =-«« + ! ^.
The total charge of electricity on the conductor is BQ.
The surface-density at any point of the sphere may be found
from the equation
dV dU
4 770- = -= -- -y-
dr dr
iYi. (73)
Distribution of Electricity on a nearly Spherical Conductor.
Let the equation of the surface of the conductor be
r = a(l+JF), (74)
146.] NEARLY SPHERICAL CONDUCTOR. 179
where F is a function of the direction of r, and is a numerical
quantity the square of which may be neglected.
Let the potential due to the external electrified system be ex
pressed, as before, in a series of solid harmonics of positive degree,
and let the potential U be a series of solid harmonics of negative
degree. Then the potential at the surface of the conductor is
obtained by substituting the value of r from equation (74) in these
series.
Hence, if C is the value of the potential of the conductor and
.Z?0 the charge upon it,
C= 4,
..-(j+l)B,arWFYr (75)
Since F is very small compared with unity, we have first a set
of equations of the form (72), with the additional equation
0 = -£Q-F + 3A1aFY1 + 8tc. + (i+l)AiaiFYi
+ 2(.#/0-U+1>7,)-2 ((j+VSja-U+VFYj). (76)
To solve this equation we must expand F, FY1 . . . FYi in terms of
spherical harmonics. If F can be expanded in terms of spherical
harmonics of degrees lower than Jc} then FYi can be expanded in
spherical harmonics of degrees lower than i + k.
Let therefore
BQ - F- 3A1aFY1- ...-(2i+l)Ai «W<= 2 (Bj a-U+DJ}), (77)
d
then the coefficients Bj will each of them be small compared with
the coefficients BQ ... Bi on account of the smallness of F, and
therefore the last term of equation (76), consisting of terms in BjF,
may be neglected.
Hence the coefficients of the form Bj may be found by expanding
equation (76) in spherical harmonics.
For example, let the body have a charge _Z?0, and be acted on by
no external force.
Let F be expanded in a series of the form
F = S1Yl + &c. + StYlk. (78)
Then £0±SlYl + &c. + £0±81gYt = '2(SJa-V+VYj), (79)
N 2
180 SPHEEICAL HAKMONICS.
or the potential at any point outside the body is
(80)
and if o- is the surface-density at any point
dU
4-770- = -- — >
dr
or 47700- = £0(l+fl2ra+...+ (£-1)^7,). (81)
Hence, if the surface differs from that of a sphere by a thin
stratum whose depth varies according to the values of a spherical
harmonic of degree /£, the ratio of the difference of the superficial
densities at any two points to their sum will be k— I times the
ratio of the difference of the radii of the same two points to their
sum.
CHAPTER X.
CONTOCAL QUADRIC SURFACES*.
147.] Let the general equation of a confocal system be
~ 2
where X is a variable parameter, which we shall distinguish by the
suffix A1 for the hyperboloids of two sheets, A,2 for the hyperboloids
of one sheet, and A3 for the ellipsoids. The quantities
0, A15 b, \.2, c, A3
are in ascending order of magnitude. The quantity a is introduced
for the sake of symmetry, but in our results we shall always suppose
a = 0.
If we consider the three surfaces whose parameters are A15 A2, A3,
we find, by elimination between their equations, that the value of
x2 at their point of intersection satisfies the equation
X*(6Z-a*)(C*-a*) = (A12-«2)(A22-«2)(A32-«2). (2)
The values of f and z2 may be found by transposing a, b, c
symmetrically.
Differentiating this equation with respect to \ly we find
dx Aj /3x
~~T — = r~9 - 9 •*• ^ '
d\± Aj2— a2
If ds^ is the length of the intercept of the curve of intersection of
A2 and A3 cut off between the surfaces Ax and Aj + ^A^ then
-^J2 'jb 2 ^ ~di\* ^ ~di\2 __ A12(A22-A12)(A32-A12)
3^1 =^XT h^| h^r! B w-^w-^xv-^)-'
* This investigation is chiefly borrowed from a very interesting work, — Lemons sur
les Fonctions Inverses des Transcendantes et les Surfaces Isotherme*. Par G.
Paris, 1857.
182 CONFOCAL QUADRIC SURFACES.
The denominator of this fraction is the product of the squares of
the semi-axes of the surface Ax .
If we put
7)2 _ \ 2 A 2 7)2 _ A 2 \ 2 <)nA 7) 2 _ \ 2 _ \ 2 /K\
•**1 — A3 — A2 ' ^2 — A3 — Al J anQ -^3 — A2 — Al > \P)
and if we make a = 0, then
d _ D2D3 (
It is easy to see that Z^2 and D3 are the semi-axes of the central
section of Ax which is conjugate to the diameter passing1 through
the given point, and that D2 is parallel to ds2, and D3 to ds3.
If we also substitute for the three parameters \lt A2, A3 their
values in terms of three functions a, (3, y, denned by the equations
da c .
-j — = , — . > A., = 0 when a = 0,
/ 2 /2 /^f =1 > A2 = * When £ = °> (7)
VA22 — b2Vc2 — A22
/?
A3 = c when y = 0 ;
then ^ = -D2D3 da, ds2 = -D3D1 dp, ds3 — -D^D^ dy. (8)
0 C 0
148.] Now let V be the potential at any point a, /3, y, then the
resultant force in the direction of ds is
__ _L_ dV c
1- ds[" JadSl~ "Jal)^!^'
Since ^, ds2, and ^3 are at right angles to each other, the
surface-integral over the element of area ds2 ds3 is
- dV c DD D,D
Now consider the element of volume intercepted between the
surfaces a, /3, y, and a + ^a, fi + dfa y + dy. There will be eight
such elements^ one in each octant of space.
We have found the surface-integral for the element of surface
intercepted from the surface a by the surfaces (3 and p + dfi, y and
I49-] TRANSFORMATION OF POISSON's EQUATION. 183
The surface-integral for the corresponding element of the surface
a-f da will be
da c
since D^ is independent of a. The surface-integral for the two
opposite faces of the element of volume, taken with respect to the
interior of that volume, will be the difference of these quantities, or
Similarly the surface-integrals for the other two pairs of forces
will be
. and
c dy2 c
These six faces enclose an element whose volume is
727 2 7 2
and if p is the volume-density within that element, we find by
Art. 77 that the total surface-integral of the element, together with
the quantity of electricity within it, multiplied by 4 TT is zero, or,
dividing by dadfidy,
which is the form of Poisson's extension of Laplace's equation re
ferred to ellipsoidal coordinates.
If p = 0 the fourth term vanishes, and the equation is equivalent
to that of Laplace.
For the general discussion of this equation the reader is referred
to the work of Lame already mentioned.
149.] To determine the quantities a, 0, y, we may put them in
the form of ordinary elliptic functions by introducing the auxiliary
angles 0, $, and \//-, where
Ax = £sin0,
A2 = V c2 sin2 $ + b2 cos^), (13)
sm\//
If we put 5 = h, and F + £/2 - 1, we may call k and It the two
complementary moduli of the confocal system, and we find
184 CONFOCAL QUADKIC SURFACES. [l 50.
an elliptic integral of the first kind, which we may write according
to the usual notation F(kO}.
In the same way we find
13 =
, 2^
1— /£ 2 cos2</>
where FJc is the complete function for modulus k' ',
y •* 7 o * o "•
V 1 — k* sm2 \lr
Here a is represented as a function of the angle 0, which is a
function of the parameter A15 /3 as a function of </> and thence of A2,
and y as a function of \j/ and thence of A3 .
But these angles and parameters may be considered as functions
of a, (3, y. The properties of such inverse functions, and of those
connected with them, are explained in the treatise of M. Lame on
that subject.
It is easy to see that since the parameters are periodic functions
of the auxiliary angles, they will be periodic functions of the
quantities a, /3, y : the periods of Aj and A3 are 4 F(k) and that of A2
is 2 F(Jc').
Particular Solutions.
150.] If V is a linear function of a, (3, or y, the equation is
satisfied. Hence we may deduce from the equation the distribution
of electricity on any two confocal surfaces of the same family
maintained at given potentials, and the potential at any point
between them.
The Hyperboloids of Two Sheets.
When a is constant the corresponding surface is a hyperboloid
of two sheets. Let us make the sign of a the same as that of x in
the sheet under consideration. We shall thus be able to study one
of these sheets at a time.
Let ax , a2 be the values of a corresponding to two single sheets,
whether of different hyperboloids or of the same one, and let F15 F2
be the potentials at which they are maintained. Then, if we make
-a2ri + a(77i— Tg) fio\
, (18)
the conditions will be satisfied at the two surfaces and throughout
the space between them. If we make V constant and. equal to V±
in the space beyond the surface al5 and constant and equal to F2
150.] DISTRIBUTION OF ELECTRICITY. 185
in the space beyond the surface a2, we shall have obtained the
complete solution of this particular case.
The resultant force at any point of either sheet is
R _ _dF_ _dFda
dsl ~ da ds±
or ^ = ri~r2 C . (20)
If pi be the perpendicular from the centre on the tangent plane
at any point, and Pl the product of the semi-axes of the surface,
then plD2 D.3 = P1.
Hence we find „ ^1—^2 CP\ ^oi\
1 = a — ~p~
or the force at any point of the surface is proportional to the per
pendicular from the centre on the tangent plane.
The surface-density a- may be found from the equation
4-770- = ^. (22)
The total quantity of electricity on a segment cut off by a plane
whose equation is x = a from one sheet of the hyperboloid is
£--iV (23)
2 al-a2
The quantity on the whole infinite sheet is therefore infinite.
The limiting forms of the surface are : —
(1) When a = F^ the surface is the part of the plane of xz on
the positive side of the positive branch of the hyperbola whose
equation is #2 zz
To o := 1 • \ /
b2 c2
(2) When a = 0 the surface is the plane of yz.
(3) When a = —F^ the surface is the part of the plane of xz on
the negative side of the negative branch of the same hyperbola.
The Hyperloloids of One Sheet.
By making /3 constant we obtain the equation of the hyperboloid
of one sheet. The two surfaces which form the boundaries of the
electric field must therefore belong to two different hyperboloids.
The investigation will in other respects be the same as for the
hyperboloids of two sheets, and when the difference of potentials
is given the density at any point of the surface will be proportional
to the perpendicular from the centre on the tangent plane, and the
whole quantity on the infinite sheet will be infinite.
186 CONFOCAL QUADRIC SURFACES.
Limiting Forms.
(1) When /3 = 0 the surface is the part of the plane of xz
between the two branches of the hyperbola whose equation is
written above, (24).
(2) When £ = F(k') the surface is the part of the plane of xy
which is on the outside of the focal ellipse whose equation is
The Ellipsoids.
For any given ellipsoid y is constant. If two ellipsoids, y: and y2,
be maintained at potentials V^ and V^ then, for any point y in the
space between them, we have
^) (26)
71 — 72
The surface-density at any point is
where p3 is the perpendicular from the centre on the tangent plane,
and P3 is the product of the semi-axes.
The whole charge of electricity on either surface is
a finite quantity.
When y = F(k) the surface of the ellipsoid is at an infinite
distance in all directions.
If we make V2 = 0 and y2 = F(k), we find for the quantity of
electricity on an ellipsoid maintained at potential V in an infinitely
extended field, V , .
^ — c WTr\ — v • /
F(k)*-y
The limiting form of the ellipsoids occurs when y = 0, in which
case the surface is the part of the plane of xy within the focal
ellipse, whose equation is written above. (25).
The surface-density on the elliptic plate whose equation is (25), and
whose eccentricity is $, is
o- = - V — __ X — , (30)
/
V
and its charge is _ V
151.] SURFACES OF REVOLUTION. 187
Particular Cases.
151.] If k is diminished till it becomes ultimately zero, the
system of surfaces becomes transformed in the following manner : —
The real axis and one of the imaginary axes of each of the
hyperboloids of two sheets are indefinitely diminished, and the
surface ultimately coincides with two planes intersecting in the
axis of z.
The quantity a becomes identical with 6, and the equation of the
system of meridional planes to which the first system is reduced is
1-2 ,,,2
_? ?. = o. (32)
(sin a)2 (cos a)2
The quantity /3 is reduced to
^=/s-£ = lo^tant' (33)
whence we find
2
smd) = -5 - 5> cosc> =
e* -f e~
If we call the exponential quantity \(e^ + er^) the hyperbolic
cosine of /3, or more concisely the hypocosine of /3, or cos h ft, and if
we call i (e^ — e-P} the hyposine of ft, or sin^ ft, and if by the same
analogy we call
the hyposecant of ft, or sec h ft,
cos h ft
1
sin^/3
sin hf$
cost ft
the hypocosecant of ft, or cosec Ji ft,
the hypotangent of ft, or tan h ft,
and COS 1P the hypoeotang-ent of ft, or cot Ji ft ;
sm/fc/3
then A2 = c sec h ft, and the equation of the system of hyperboloids
of one sheet is
= C2 (35)
"
(seek ft)'2
The quantity y is reduced to \ff, so that A3 = c cosec y, and the
equation of the system of ellipsoids is
O O 9
* +y + * = C2. (36)
(secy)2 (tany)2
Ellipsoids of this kind, which are figures of revolution about their
conjugate axes, are called Planetary ellipsoids.
188 CONFOCAL QUADRIC SURFACES.
The quantity of electricity on a planetary ellipsoid maintained at
potential V in an infinite field, is
gastf-JL-, (37)
where c sec y is the equatorial radius, and c tan y is the polar radius.
If y = 0, the figure is a circular disk of radius c, and
V
a = -- . (38)
W*,v/e2-ra
Q = c~ (39)
2
152.] to0»^ Ciw*. Let b — c, then £ = 1 and £' = 0,
_ 9$
a = log tan - , whence Ax = c tan ^ a, (40)
and the equation of the hyperboloids of revolution of two sheets
becomes #2 ^2
(sec/*a)2 "
The quantity /3 becomes reduced to <£, and each of the hyper
boloids of one sheet is reduced to a pair of planes intersecting in
the axis of x whose equation is
(sin/3)2 (cos/3)2
This is a system of meridional planes in which (3 is the longitude.
The quantity y becomes log tan -- — 5 whence A3 = c cot k y,
and the equation of the family of ellipsoids is
>2 .,2 I «2
(cosec/5y)2~
These ellipsoids, in which the transverse axis is the axis of revo-
lution, are called Ovary ellipsoids.
The quantity of electricity on an ovary ellipsoid maintained at a
potential V in an infinite field is
Q = c — • (44)
If the polar radius is A = c cot h y, and the equatorial radius is
B = c cosec Ji y,
- /AK.
= log -- --- (45)
1 54.] CYLINDERS AND PARABOLOIDS. 189
If the equatorial radius is very small compared to the polar radius,
as in a wire with rounded ends,
y = log-,, j and Q = ; -A — ; 5- (46)
& B log A— logjfj
When both b and c become zero, their ratio remaining finite,
the system of surfaces becomes two systems of confocal cones, and
a system of spherical surfaces of which the radius is inversely
proportional to y.
If the ratio of b to c is zero or unity, the system of surfaces
becomes one system of meridian planes, one system of right cones
having a common axis, and a system of concentric spherical surfaces
of which the radius is inversely proportional to y. This is the
ordinary system of spherical polar coordinates.
Cylindric Surfaces.
153.] When c is infinite the surfaces are cylindric, the generating
lines being parallel to z. One system of cylinders is elliptic, with
the equation
/yi2 ;/ -
x + __! = £2 (47)
(cos/fcci)2 ^ (sin/U)2
The other is hyperbolic, with the equation
-r2 <?/2
y _ £2 (48)
(cos/3)2 (sin/3)2"
This system is represented in Fig. X, at the end of this volume.
Confocal Paraboloids.
154.] If in the general equations we transfer the origin of co
ordinates to a point on the axis of x distant t from the centre of
the system, and if we substitute for x, A, d, and c, t + x, 1 + A, l + b,
and l + c respectively, and then make I increase indefinitely, we
obtain, in the limit, the equation of a system of paraboloids whose
foci are at the points x = b and x = c,
If the variable parameter is A for the first system of elliptic
paraboloids, JJL for the hyperbolic paraboloids, and v for the second
system of elliptic paraboloids, we have A, 5, /u, c, v in ascending
order of magnitude, and
190
CONFOCAL QUADRIC SUltFACES.
[I54-
x =
c-b
z*
~~c=b~
\ = \ (b + c) — \(c — b) cos ha,
x —
= 2 (c— b) sin^- sin -
v ' 22
— b) (cos fry — cos/3— cos /i a), "
1
2'
— %(c— b)cosh -cos- sin ^ -
LI LI &
(50)
(51)
(52)
When 5 = c we have the case of paraboloids of revolution about
the axis of a?, and x •= a (e^—e2^,
y = 2aea+y cos/3, (53)
The surfaces for which /3 is constant are planes through the axis,
/3 being the angle which such a plane makes with a fixed plane
through the axis.
The surfaces for which a is constant are confocal paraboloids.
When a=0 the paraboloid is reduced to a straight line terminating
at the origin.
We may also find the values of a, ft, y in terms of r, 6, and $,
the spherical polar coordinates referred to the focus as origin,, and
the axis of the parabolas as axis of the sphere,
a = log
cos i 0),
ft = <P> (54)
y = log (f* sin 4 &}•
We may compare the case in which the potential is equal to a,
with the zonal solid harmonic ri Q4. Both satisfy Laplace's equa
tion, and are homogeneous functions of x, y, #, but in the case
derived from the paraboloid there is a discontinuity at the axis, and
i has a value not differing by any finite quantity from zero.
The surface-density on an electrified paraboloid in an infinite
field (including the case of a straight line infinite in one direction)
is inversely as the distance from the focus, or, in the case of
the line, from the extremity of the line.
CHAPTER XL
THEORY OF ELECTRIC IMAGES AND ELECTRIC INVERSION.
155.] We have already shewn that when a conducting sphere
is under the influence of a known distribution of electricity, the
distribution of electricity on the surface of the sphere can be
determined by the method of spherical harmonics.
For this purpose we require to expand the potential of the in
fluencing system in a series of solid harmonics of positive degree,
having the centre of the sphere as origin, and we then find a
corresponding series of solid harmonics of negative degree, which
express the potential due to the electrification of the sphere.
By the use of this very powerful method of analysis, Poisson
determined the electrification of a sphere under the influence of
a given electrical system, and he also solved the more difficult
problem to determine the distribution of electricity on two con
ducting spheres in presence of each other. These investigations
have been pursued at great length by Plana and others, who have
confirmed the accuracy of Poisson.
In applying this method to the most elementary case of a sphere
under the influence of a single electrified point, we require to expand
the potential due to the electrified point in a series of solid har
monics, and to determine a second series of solid harmonics which
express the potential, due to the electrification of the sphere, in the
space outside.
It does not appear that any of these mathematicians observed
that this second series expresses the potential due to an imaginary
electrified point, which has no physical existence as an electrified
point, but which may be called an electrical image, because the
action of the surface on external points is the same as that which
would be produced by the imaginary electrified point if the spherical
surface were removed.
192 ELECTKIC IMAGES. [156.
This discovery seems to have been reserved for Sir W. Thomson,
who has developed it into a method of great power for the solution
of electrical problems, and at the same time capable of being pre
sented in an elementary geometrical form.
His original investigations, which are contained in the Cambridge
and Dublin Mathematical Journal, 1848, are expressed in terms of
the ordinary theory of attraction at a distance, and make no use of
the method of potentials and of the general theorems of Chapter IV,
though they were probably discovered by these methods. Instead,
however, of following the method of the author, I shall make free
use of the idea of the potential and of equipotential surfaces, when
ever the investigation can be rendered more intelligible by such
means.
Theory of Electric Images.
156.] Let A and B, Figure 7, represent two points in a uniform
dielectric medium of infinite extent.
Let the charges of A and B be e1
and e2 respectively. Let P be any
point in space whose distances from
A and B are r^ and r2 respectively.
Then the value of the potential at P
will be TT e\ e2
Fig. 7. TI, r*
The equipotential surfaces due to
this distribution of electricity are represented in Fig. I (at the end
of this volume) when e1 and e2 are of the same sign, and in Fig. II
when they are of opposite signs. We have now to consider that
surface for which V = 0, which is the only spherical surface in
the system. When e1 and e2 are of the same sign, this surface is
entirely at an infinite distance, but when they are of opposite signs
there is a plane or spherical surface at a finite distance for which
the potential is zero.
The equation of this surface is
fL + .4 = 0. (2)
Its centre is at a point C in AB produced, such that
AC'.BC:-. e±2 : e22,
and the radius of the sphere is
The two points A and B are inverse points with respect to this
15 7-] INVERSE POINTS. 193
sphere, that is to say, they lie in the same radius, and the radius is
a mean proportional between their distances from the centre.
Since this spherical surface is at potential zero, if we suppose
it constructed of thiii metal and connected with the earth, there
will be no alteration of the potential at any point either outside or
inside, but the electrical action everywhere will remain that due to
the two electrified points A and B.
If we now keep the metallic shell in connexion with the earth
and remove the point £, the potential within the sphere will become
everywhere zero, but outside it will remain the same as before.
For the surface of the sphere still remains at the same potential,
and no change has been made in the exterior electrification.
Hence, if an electrified point A be placed outside a spherical
conductor which is at potential zero, the electrical action at all
points outside the sphere will be that due to the point A together
with another point B within the sphere, which we may call the
electrical image of A.
In the same way we may shew that if B is a point placed inside
the spherical shell, the electrical action within the sphere is that
due to B, together with its image A.
157.] Definition of an Electrical Image. An electrical image is
an electrified point or system of points on one side of a surface
which would produce on the other side of that surface the same
electrical action which the actual electrification of that surface
really does produce.
In Optics a point or s}Tstem of points on one side of a mirror
or lens which if it existed would emit the system of rays which
actually exists on the other side of the mirror or lens, is called a
virtual image.
Electrical images correspond to virtual images in optics in being
related to the space on the other side of the surface. They do not
correspond to them in actual position, or in the merely approximate
character of optical foci.
There are no real electrical images, that is, imaginary electrified
points which would produce, in the region on the same side of the
electrified surface, an effect equivalent to that of the electrified surface.
For if the potential in any region of space is equal to that due
to a certain electrification in the same region it must be actually
produced by that electrification. In fact, the electrification at any
point may be found from the potential near that point by the
application of Poisson's equation.
VOL. I. O
194 ELECTRIC IMAGES. [l 57 .
Let a be the radius of the sphere.
Let/* be the distance of the electrified point A from the centre C.
Let e be the charge of this point.
Then the image of the point is at S, on the same radius of the
o
sphere at a distance —= , and the charge of the image is —e-f
J J
We have shewn that this image
will produce the same effect on the
opposite side of the surface as the
actual electrification of the surface
does. We shall next determine the
surface-density of this electrifica
tion at any point P of the spherical
surface, and for this purpose we shall
make use of the theorem of Coulomb,
Art. 80, that if R is the resultant force at the surface of a conductor,
and o- the superficial density,
R = 477 0-,
R being measured away from the surface.
We may consider R as the resultant of two forces, a repulsion
Z> /Tf 1
-— =_ acting along AP, and an attraction e -, ^-^ acting along PB.
A.L J JL Jj
Resolving these forces in the directions of AC and CP, we find
that the components of the repulsion are
Pi P ft
- along AC, and along CP.
Those of the attraction are
f £P3 * BP> CP'
o a
Now BP — — r AP, and BC = — , so that the components of
J J
the attraction may be written
1 f2 I
AC> and ~e
The components of the attraction and the repulsion in the
direction of AC are equal and opposite, and therefore the resultant
force is entirely in the direction of the radius CP. This only
confirms what we have already proved, that the sphere is an equi-
potential surface, and therefore a surface to which the resultant
force is everywhere perpendicular.
158.] DISTRIBUTION OF ELECTRICITY. 195
The resultant force measured along CP, the normal to the surface
in the direction towards the side on which A is placed, is
a AP3
If A is taken inside the sphere f is less than «, and we must
measure R inwards. For this case therefore
V^A: «>
In all cases we may write
AD. Ad 1 ,^
R = -e~Cp-AP*>
where AD, Ad are the segments of any line through A cutting the
sphere, and their product is to be taken positive in all cases.
158.] From this it follows, by Coulomb's theorem, Art. 80,
that the surface-density at P is
AD. Ad 1 ,R.
*=-eI^CPAP*'
The density of the electricity at any point of the sphere varies
inversely as the cube of its distance from the point A.
The effect of this superficial distribution, together with that of
the point J, is to produce on the same side of the surface as the
point A a potential equivalent to that due to e at A, and its image
— e -p at B> and on the other side of the surface the potential is
t/
everywhere zero. Hence the effect of the superficial distribution
by itself is to produce a potential on the side of A equivalent to
that due to the image — e- at B, and on the opposite side a
J
potential equal and opposite to that of e at A.
The whole charge on the surface of the sphere is evidently — e -
since it is equivalent to the image at B.
We have therefore arrived at the following theorems on the
action of a distribution of electricity on a spherical surface, the
surface-density being inversely as the cube of the distance from
a point A either without or within the sphere.
Let the density be given by the equation
. . .
where C is some constant quantity, then by equation (6)
. (8)
196 ELECTRIC IMAGES.
The action of this superficial distribution on any point separated
from A by the surface is equal to that of a quantity of electricity
— <?, or 4 TT a C
AD .Ad
concentrated at A.
Its action on any point on the same side of the surface with A is
equal to that of a quantity of electricity
fAD.Ad
concentrated at B the image of A.
The whole quantity of electricity on the sphere is equal to the
first of these quantities if A is within the sphere, and to the second
if A is without the sphere.
These propositions were established by Sir W. Thomson in his
original geometrical investigations with reference to the distribution
of electricity on spherical conductors, to which the student ought
to refer.
159.] If a system in which the distribution of electricity is
known is placed in the neighbourhood of a conducting sphere of
radius a, which is maintained at potential zero by connexion with
the earth, then the electrifications due to the several parts of the
system will be superposed.
Let AI} A2, &c. be the electrified points of the system, f^f^ &c.
their distances from the centre of the sphere, e19 e2, &c. their
charges, then the images _Z?ls .Z?2, &c. of these points will be in the
a2 a2
same radii as the points themselves, and at distances -~- > -^- &c.
«/l e/2
from the centre of the sphere, and their charges will be
a a
-£ — , _e?--&c,
fl /2
The potential on the outside of the sphere due to the superficial
electrification will be the same as that which would be produced by
the system of images £19 H2, &c. This system is therefore called
the electrical image of the system A1, A2, &c.
If the sphere instead of being at potential zero is at potential 7?,
we must superpose a distribution of electricity on its outer surface
having the uniform surface-density
7
The effect of this at all points outside the sphere will be equal to
l6o.] IMAGE OF AN ELECTRIFIED SYSTEM. 197
that of a quantity Va of electricity placed at its centre, and at
all points inside the sphere the potential will be simply increased
by 7.
The whole charge on the sphere due to an external system of
influencing points Aly A2, &c. is
E= Fa-e"-e.,^--&e., (9)
/I /2
from which either the charge E or the potential V may be cal
culated when the other is given.
When the electrified system is within the spherical surface the
induced charge on the surface is equal and of opposite sign to the
inducing charge, as we have before proved it to be for every closed
surface, with respect to points within it.
160.] The energy due to the mutual action between an elec
trified point e, at a distance f from the centre of the sphere greater
than a the radius, and the electrification of the spherical surface
due to the influence of the electrified point and the charge of the
sphere, is
where Fis the potential, and .2? the charge of the sphere.
The repulsion between the electrified point and the sphere is
therefore, by Art. 92,
,V ef .
Hence the force between the point and the sphere is always an
attraction in the following cases —
(1) When the sphere is uninsulated.
(2) When the sphere has no charge.
(3) When the electrified point is very near the surface.
In order that the force may be repulsive, the potential of the
/'3
sphere must be positive and greater than e ' — ^ , and the
charge of the sphere must be of the same sign as e and greater
At the point of equilibrium the equilibrium is unstable, the force
198 ELECTRIC IMAGES. [l6l.
being an attraction when the bodies are nearer and a repulsion
when they are farther off.
When the electrified point is within the spherical surface the
force on the electrified point is always away from the centre of
the sphere, and is equal to
The surface-density at the point of the sphere nearest to the
electrified point where it lies outside the sphere is
The surface-density at the point of the sphere farthest from the
electrified point is
When E, the charge of the sphere, lies between
«W-«) nd
Af+
the electrification will be negative next the electrified point and
positive on the opposite side. There will be a circular line of division
between the positively and the negatively electrified parts of the
surface, and this line will be a line of equilibrium.
If E= e
the equipotential surface which cuts the sphere in the line of equi
librium is a sphere whose centre is the electrified point and whose
radius is \/f2 — a2.
The lines of force and equipotential surfaces belonging to a case
of this kind are given in Figure IV at the end of this volume.
Images in an Infinite Plane Conducting Surface.
161.] If the two electrified points A and B in Art. 156 are
electrified with equal charges of electricity of opposite signs, the
surface of zero potential will be the plane, every point of which is
equidistant from A and B.
1 62.]
IMAGES IN AN INFINITE PLANE.
199
Hence, if A be an electrified point whose charge is e, and AD
a perpendicular on the plane, produce AD
to B so that D£ = ^£, and place at B
a charge equal to —e, then this charge
at B will be the image of A} and will
produce at all points on the same side of
the plane as A, an effect equal to that
of the actual electrification of the plane.
For the potential on the side of A due
to A and B fulfils the conditions that
V2F= 0 everywhere except at A, and
that V = 0 at the plane, and there is only Fig. 8.
one form of V which can fulfil these conditions.
To determine the resultant force at the point P of the plane, we
M
observe that it is compounded of two forces each equal to --^ ,
one acting along AP and the other along PB. Hence the resultant
of these forces is in a direction parallel to AB and equal to
Hence R, the resultant force measured from the surface towards the
space in which A lies, is
and the density at the point P is
eAD
On Electrical Inversion.
162.] The method of electrical images leads directly to a method
of transformation by which we may derive from any electrical
problem of which we know the solution any number of other
problems with their solutions.
We have seen that the image of a point at a distance r from the
centre of a sphere of radius R is in the same radius and at a distance
/ such that r/=R2. Hence the image of a system of points, lines,
or surfaces is obtained from the original system by the method
known in pure geometry as the method of inversion, and described
by Chasles, Salmon, and other mathematicians.
200 ELECT1UC IMAGES. [162.
If A and B are two points, A' and B' their images, 0 being the
A centre of inversion, and R the radius of the
sphere of inversion,
OA.OA' = R2 = OB.OB'.
Hence the triangles OAB} OB'A' are similar,
and AB : A'B' : : 0 A : OBf : : OA.OB : R2.
B'
Fi£- 9> If a quantity of electricity e be placed at A,
its potential at B will be e
= AS '
If e' be placed at A' its potential at B' will be
V'=~IW
In the theory of electrical images
e:e'::OA:R::R: OA'.
Hence V \V :: R:OB, (17)
or the potential at B due to the electricity at A is to the potential
at the image of B due to the electrical image of A as R is to OB.
Since this ratio depends only on OB and not on OA, the potential
at B due to any system of electrified bodies is to that at B' due
to the image of the system as R is to OB.
If r be the distance of any point A from the centre, and / that
of its image A', and if e be the electrification of A, and / that of A'}
also if L, 8, K be linear, superficial, and solid elements at A, and
If, S', K' their images at A', and A, <r, p, A', </, p' the corresponding
line-surface and volume-densities of electricity at the two points,
V the potential at A due to the original system, 'and V the potential
at A' due to the inverse system, then
/ _ L' _ R2 _ /a S' _ ^4 _ /4 K' _R* _ /6
"7 := T " 72~~ ~~.P' 1T=~7¥~~ ^*' If ~~ rr ~" ]£*
_/ JS ^ A^_ ^__-#
~~e ~" ~r '~~~R' T "" ^ "" / '
(/ r3 .S3 p' r5 _ 725
(T ^3 /^ p .S5 "~ r'5
V _ r _ R
T -Jf==V
If in the original system a certain surface is that of a conductor,
* See Thomson and Tait's Natural Philosophy, § 515.
163.] GEOMETRICAL THEOREMS. 201
and has therefore a constant potential P, then in the transformed
T)
system the image of the surface will have a potential P -^ - But
by placing at 0, the centre of inversion, a quantity of electricity
equal to — PR, the potential of the transformed surface is reduced
to zero.
Hence, if we know the distribution of electricity on a conductor
when insulated in open space and charged to the potential P, we
can find by inversion the distribution on a conductor whose form is
the image of the first under the influence of an electrified point with
a charge —PR placed at the centre of inversion, the conductor
being in connexion with the earth.
163.] The following geometrical theorems are useful in studying
cases of inversion.
Every sphere becomes, when inverted, another sphere, unless
it passes through the centre of inversion, in which case it becomes
a plane.
If the distances of the centres of the spheres from the centre of
inversion are a and a', and if their radii are a and a', and if we
define the power of the sphere with respect to the centre of in
version to be the product of the segments cut off by the sphere
from a line through the centre of inversion, then the power of the
first sphere is cr — a2, and that of the second is a'2 — a'2. We
have in this case
a' a' R2 a?*-^
- = — =- 8s=— 3=— i (19)
a a a-' — a- £F
or the ratio of the distances of the centres of the first and second
spheres is equal to the ratio of their radii, and to the ratio of the
power of the sphere of inversion to the power of the first sphere,
or of the power of the second sphere to the power of the sphere
of inversion.
The centre of either sphere corresponds to the inverse point of
the other with respect to the centre of inversion.
In the case in which the inverse surfaces are a plane and a
sphere, the perpendicular from the centre of inversion on the plane
is to the radius of inversion as this radius is to the diameter of
the sphere, and the sphere has its centre on this perpendicular and
passes through the centre of inversion.
Every circle is inverted into another circle unless it passes
through the centre of inversion, in which case it becomes a straight
line.
202 ELECTRIC IMAGES. [164.
The angle between two surfaces, or two lines at their intersection,
is not changed by inversion.
Every circle which passes through a point, and the image of that
point with respect to a sphere, cuts the sphere at right angles.
Hence, any circle which passes through a point and cuts the
sphere at right angles passes through the image of the point.
164.] "We may apply the method of inversion to deduce the
distribution of electricity on an uninsulated sphere under the
influence of an electrified point from the uniform distribution on
an insulated sphere not influenced by any other body.
If the electrified point be at A, take it for the centre of inversion,
and if A is at a distance f from the centre of the sphere whose
radius is #, the inverted figure will be a sphere whose radius is a'
and whose centre is distant f\ where
a' f R2 (20}
=='
The centre of either of these spheres corresponds to the inverse
point of the other with respect to A, or if C is the centre and B the
inverse point of the first sphere, C' will be the inverse point, and B'
the centre of the second.
Now let a quantity e of electricity be communicated to the
second sphere, and let it be uninfluenced by external forces. It
will become uniformly distributed over the sphere with a surface-
density ^
Its action at any point outside the sphere will be the same as
that of a charge e placed at B' the centre of the sphere.
At the spherical surface and within it the potential is
P' = 7 ' (22)
a constant quantity.
Now let us invert this system. The centre B' becomes in the
inverted system the inverse point B, and the charge e' at B'
AB
becomes ^ -JT~ at B, and at any point separated from B by the
surface the potential is that due to this charge at B.
The potential at any point P on the spherical surface, or on the
same side as _Z?, is in the inverted system
AP
165.] SYSTEMS OF IMAGES. 203
If we now superpose on this system a charge e at A, where
the potential on the spherical surface, and at all points on the same
side as B, will be reduced to zero. At all points on the same side
as A the potential will be that due to a charge e at A, and a charge
e'~ at B.
But e'jT=-eaj7 = -ear (24)
as we found before for the charge of the image at B.
To find the density at any point of the first sphere we have
<»»>
Substituting for the value of a-' in terms of the quantities be
longing to the first sphere, we find the same value as in Art. 158,
22 (26)
V /
On Finite Systems of Successive Images.
165.] If two conducting planes intersect at an angle which is
a submultiple of two right angles, there will be a finite system of
images which will completely determine the electrification.
For let AOB be a section of the two conducting planes per
pendicular to their line of inter
section, and let the angle of
intersection AOB = -, let P
n
be an electrified- point, and let
PO = r, and POB = 0. Then,
if we draw a circle with centre 0
and radius OP, and find points
which are the successive images
of P in the two planes beginning
with OB, we shall find Qx for the Fig. 10.
image of P in OB, P2 for the image of Ql in OA, Q3 for that of P2
in OB, P3 for that of Q3 in OA, and Q.2 for that of P3 in OB.
If we had begun with the image of P in AO we should have
found the same points in the reverse order Q2, P3, Q3, P2, Q19
provided AOB is a submultiple of two right angles.
204 ELECTRIC IMAGES. [l66.
For the alternate images P15 P2, P3 are ranged round the circle
at angular intervals equal to 2 AOB, and the intermediate images
Qi> 625 QB are a^ intervals of the same magnitude. Hence, if
2 AOB is a sub multiple of 27r, there will be a finite number of
images, and none of these will fall within the angle AOB. If,
however, AOB is not a submultiple of it, it will be impossible to
represent the actual electrification as the result of a finite series of
electrified points.
If AOB = -, there will be n negative images Qlt Q2) &c., each
equal and of opposite sign to P, and n — 1 positive images P2,
P3, &c., each equal to P, and of the same sign.
The angle between successive images of the same sign is
fv
If we consider either of the conducting planes as a plane of sym
metry, we shall find the positive and negative images placed
symmetrically with regard to that plane, so that for every positive
image there is a negative image in the same normal, and at an
equal distance on the opposite side of the plane.
If we now invert this system with respect to any point, the two
planes become two spheres, or a sphere and a plane intersecting
at an angle - , the influencing point P being within this angle.
p§
The successive images lie on the circle which passes through P
and intersects both spheres at right angles.
To find the position of the images we may either make use of
the principle that a point and its image are in the same radius
of the sphere, and draw successive chords of the circle beginning
at P and passing through the centres of the two spheres al
ternately.
To find the charge which must be attributed to each image, take
any point in the circle of intersection, then the charge of each
image is proportional to its distance from this point, and its sign
is positive or negative according as it belongs to the first or the
second system.
166.] We have thus found the distribution of the images when
any space bounded by a conductor consisting of two spherical surfaces
meeting at an angle - , and kept at potential zero, is influenced by
an electrified point.
We may by inversion deduce the case of a conductor consisting
1 66.] TWO INTERSECTING SPHERES. 205
o/ two spherical segments meeting at a re-entering angle - , charged
to potential unity and placed in free space.
For this purpose we invert the system with respect to P. The
circle on which the images formerly lay now becomes a straight
line through the centres of the spheres.
If the figure (11) represents
a section through the line of
centres AB, and if D, J/ are the
points where the circle of in
tersection cuts the plane of the
paper, then, to find the suc
cessive images, draw DA a
radius of the first circle, and
draw DC, DB, &i?., making
7T 2lT
angles-, — , &c. with DA. Fig. 11.
n n
The points C, B, &c. at which they cut the line of centres will
he the positions of the positive images, and the charge of each
will be represented by its distances from D. The last of these
images will be at the centre of the second circle.
To find the negative images draw DP, DQ, &c., making angles
- > — , &c. with the line of centres. The intersections of these
lines with the line of centres will give the positions of the negative
images, and the charge of each will be represented by its distance
from D.
The surface-density at any point of either sphere is the sum
of the surface-densities due to the system of images. For instance,
the surface-density at any point S of the sphere whose centre is
A, is
where A, B, C, &c. are the positive series of images.
When S is on the circle of intersection the density is zero.
To find the total charge on each of the spherical segments, we
may find the surface-integral of the induction through that segment
due to each of the images.
The total charge on the segment whose centre is A due to the
image at A whose charge is DA is
206 ELECTRIC IMAGES.
where 0 is the centre of the circle of intersection.
In the same way the charge on the same segment due to the
image at B is \ (DB-\- OB), and so on, lines such as OB measured
from 0 to the left being reckoned negative.
Hence the total charge on the segment whose centre is A is
i (DA + DB + DC+ &c.) + i (OA + OB + 0(7+ &c.),
167.] The method of electrical images may be applied to any
space bounded by plane or spherical surfaces all of which cut one
another in angles which are submultiples of two right angles.
In order that such a system of spherical surfaces may exist, every
solid angle of the figure must be trihedral, and two of its angles
must be right angles, and the third either a right angle or a
submultiple of two right angles.
Hence the cases in which the number of images is finite are —
(1) A single spherical surface or a plane.
(2) Two planes, a sphere and a plane, or two spheres intersecting
at an angle - •
(3) These two surfaces with a third, which may be either plane
or spherical, cutting both orthogonally.
(4) These three surfaces with a fourth cutting the first two
orthogonally and the third at an angle , . Of these four surfaces
one at least must be spherical.
We have already examined the first and second cases. In the
first case we have a single image. In the second case we have
2^—1 images arranged in two series in a circle which passes
through the influencing point and is orthogonal to both surfaces.
In the third case we have, besides these images, their images with
respect to the third surface, that is, 4^—1 images in all besides the
influencing point.
In the fourth case we first draw through the influencing point
a circle orthogonal to the first two surfaces, and determine on it
the positions and magnitudes of the n negative images and the
n — \ positive images. Then through each of these 2n points,
including the influencing point, we draw a circle orthogonal to
the third and fourth surfaces, and determine on it two series of
i68.]
TWO SPHERES CUTTING ORTHOGONALLY.
207
images, n' in each series. We shall obtain in this way, besides the
influencing point, 2nn' — 1 positive and 2 n n negative images.
These 4 n n' points are the intersections of n circles with n' other
circles, and these circles belong to the two systems of lines of
curvature of a cy elide.
If each of these points is charged with the proper quantity of
electricity, the surface whose potential is zero will consist of n-\-n'
spheres, forming two series of which the successive spheres of the
first set intersect at angles - , and those of the second set at angles
— , while every sphere of the first set is orthogonal to every sphere
of the second set.
Case of Two Spheres cutting Orthogonally. See Fig. IV at the
end of this volume.
168.] Let A and B, Fig. 12, be the centres of two spheres cutting
each other orthogonally in D and
I/, and let the straight line DJ/ cut
the line of centres in C. Then C
is the image of A with respect to
the sphere B, and also the image
of B with respect to the sphere
whose centre is A. If AD = a,
BD = /3, then AB = \/a2 + /32, and
if we place at A} B, C quantities Fig. 12.
of electricity equal to a, /3, and ^ respectively, then both
spheres will be equipotential surfaces whose potential is unity.
We may therefore determine from this system the distribution of
electricity in the following cases :
(1) On the conductor PDQD* formed of the larger segments of
both spheres. Its potential is 1, and its charge is
a/3
+ /3 —
= AD+BD-CD.
This quantity therefore measures the capacity of such a figure
when free from the inductive action of other bodies.
The density at any point P of the sphere whose centre is A, and
the density at any point Q of the sphere whose centre is B, are
respectively
208 ELECTRIC IMAGES. [l68.
At the points of intersection, D, J/, the density is zero.
If one of the spheres is very much larger than the other, the
density at the vertex of the smaller sphere is ultimately three times
that at the vertex of the larger sphere.
(2) The lens P'DQ'L' formed by the two smaller segments of
the spheres, charged with a quantity of electricity = -. ,
Vaa-f-/32
and acted on hy points A and B, charged with quantities a and /3,
is also at potential unity, and the density at any point is expressed
by the same formulae.
(3) The meniscus DPD'Q' formed by the difference of the
segments charged with a quantity a, and acted on by points B
and C, charged respectively with quantities /3 and , is also
va2 -f /32
in equilibrium at potential unity.
(4) The other meniscus QDP'Z/ under the action of A and C.
We may also deduce the distribution of electricity on the following
internal surfaces.
The hollow lens PDQD under the influence of the internal
electrified point C at the centre of the circle DD' .
The hollow meniscus under the influence of a point at the centre
of the concave surface.
The hollow formed of the two larger segments of both spheres
under the influence of the three points A, B, C.
But, instead of working out the solutions of these cases, we shall
apply the principle of electrical images to determine the density
of the electricity induced at the point P of the external surface of
the conductor PDQI/ by the action of a point at 0 charged with
unit of electricity.
Let OA = a, OB = 6, OP = r, BP =p,
AD = a, BD= (3, AB = Jo* + pP.
Invert the system with respect to a sphere of radius unity and
centre 0.
The two spheres will remain spheres, cutting each other ortho
gonally, and having their centres in the same radii with A and B.
If we indicate by accented letters the quantities corresponding to
the inverted system,
a I , a /3
' =
1 68.] TWO SPHERES CUTTING ORTHOGONALLY. 209
If, in the inverted system, the potential of the surface is unity,
then the density at the point Pf is
If, in the original system, the density at P is <r, then
and the potential is - . By placing at 0 a negative charge of
electricity equal to unity, the potential will become zero over the
surface, and the density at P will be
4 TT ar
This gives the distribution of electricity on one of the spherical
surfaces due to a charge placed at 0. The distribution on the
other spherical surface may be found by exchanging a and b, a and
/3, and putting q or AQ instead of p.
To find the total charge induced on the conductor by the elec
trified point at 0, let us examine the inverted system.
In the inverted system we have a charge a at J', and /3' at ff,
/ Qfi
and a negative charge — at a point C' in the line
such that AC : CB : : a2 : /3'2.
If OA'= a', 0£'= V, OC'= c'} we find
_
Inverting this system the charges become
, tiff 1 a/3
and —
c
Hence the whole charge on the conductor due to a unit of
negative electricity at 0 is
a ., £ _ _ a£
VOL. I.
210
ELECTRIC IMAGES.
[I69.
Distribution of Electricity on Three Spherical Surfaces ivhich
Intersect at Right Angles.
169.] Let the radii of the spheres be a, £, y, then
* CA = 72 + a2 ~
Fig. 13.
Let PQR, Fig. 1 3, be the feet
of the perpendiculars from ABC
on the opposite sides of the tri
angle, and let 0 be the inter
section of perpendiculars.
Then P is the image of B in
the sphere y, and also the image
of C in the sphere /3. Also 0 is
the image of P in the sphere o.
Let charges a} /3, and y be
placed at A, B, and C.
Then the charge to be placed
at P is
/V 02 + y2
Also ^P =
sidered as the image of P, is
o/3y
, so that the charge at 0, con-
In the same way we may find the system of images which are
electrically equivalent to four spherical surfaces at potential unity
intersecting at right angles.
If the radius of the fourth sphere is 8, and if we make the charge
at the centre of this sphere = 8, then the charge at the intersection
of the line of centres of any two spheres, say a and p, with their
plane of intersection, is 1
v
O~ I r\f)
The charge at the intersection of the plane of any three centres
ABC with the perpendicular from D is
A/i+i+i
I/O.] FOUR SPHERES CUTTING ORTHOGONALLY. 211
and the charge at the intersection of the four perpendiculars is
1
V"l 1 1 f
ci2 /32 y2 82
System of Four Spheres Intersecting at Right Angles under the
Action of an Electrified Point.
170.] Let the four spheres be A,B,C,D, and let the electrified point
be 0. Draw four spheres AIt Bl} C1} D^, of which any one, Alt
passes through 0 and cuts three of the spheres, in this case B,
C, and D, at right angles. Draw six spheres (ab), (ac), (ad), (be),
(bd), (cd)} of which each passes through 0 and through the circle
of intersection of two of the original spheres.
The three spheres BI} Clt D^ will intersect in another point besides
0. Let this point be called A', and let ff, C', and I/ be the
intersections of Cl} J)lt Alf of Dlt AI} BI} and of A19 B±, C± re
spectively. Any two of these spheres, Al, B1, will intersect one of
the six (cd) in a point (ab'). There will be six such points.
Any one of the spheres, Alf will intersect three of the six (ab),
(ac), (ad) in a point a '. There will be four such points. Finally,
the six spheres (ab), (ac), (ad), (cd), (db), (be), will intersect in one
point S.
If we now invert the system with respect to a sphere of radius
R and centre 0, the four spheres A, B, C, D will be inverted into
spheres, and the other ten spheres will become planes. Of the
points of intersection the first four A', Bf, C', If will become the
centres of the spheres, and the others will correspond to the other
eleven points in the preceding article. These fifteen points form
the image of 0 in the system of four spheres.
At the point A', which is the image of 0 in the sphere A, we
must place a charge equal to the image of 0, that is, , where a
a
is the radius of the sphere A, and a is the distance of its centre
from 0. In the same way we must place the proper charges at
ff, C', I/.
The charges for each of the other eleven points may be found from
the expressions in the last article by substituting a', /3', y', 6' for
a, /3, y, 8, and multiplying the result for each point by the distance
of the point from 0, where
c2-y2
P 2
212
ELE.CTRIC IMAGES.
[171-
Two Spheres not Intersecting.
171.] When a space is bounded by two spherical surfaces which
do not intersect, the successive images of an influencing point
within this space form two infinite series, all of which lie beyond
the spherical surfaces, and therefore fulfil the condition of the
applicability of the method of electrical images.
Any two non-intersecting spheres may be inverted into two
concentric spheres by assuming as the point of inversion either
of the two common inverse points of the pair of spheres.
We shall begin, therefore, with the case of two uninsulated
concentric spherical surfaces, subject to the induction of an elec
trified point placed between them.
Let the radius of the first be b, and that of the second be1***, and
let the distance of the influencing point from the centre be r = beu.
Then all the successive images will be on the same radius as the
influencing point.
Let Q0, Fig. 14, be the image of P in the first sphere, P1 that
of QG in the second sphere, Q1 that of P1 in the first sphere, and
so on ; then
and OPS.OQS_1 = b2 e2™,
also OQ0 = de~u,
f)T> 73yjtt + 2'SJ
\JJL -I — — C/C/ *
OQ1 = fo-(»+a*n, &c.
Hence OP8 = h
OQS = *<
If the charge of P is denoted by P,
FiS-14- then
Ps = Pe>™, Qs=-Pe-(«+*™\
Next, let Q/ be the image of P in the second sphere, P/ that of
!/ in the first, &c.,
OQ,'=
OP2'=
P; = Pe~*™} Qs'= Pe^~u.
Of these images all the P's are positive, and all the Q's negative,
all the P/'s and Q's belong to the first sphere, and all the P's and
/5s to the second.
1/2.]
TWO SPHEEES NOT INTERSECTING.
213
The images within the first sphere form a converging series, the
sum of which is
-P
This therefore is the quantity of electricity on the first or interior
sphere. The images outside the second sphere form a diverging
series, but the surface-integral of each with respect to the spherical
surface is zero. The charge of electricity on the exterior spherical
surface is therefore
•e ~tt —
V*»_l )
If we substitute for these expressions their values in terms of
OA, OB, and OP, we find
charge «H —*<£
nOS AP
nJB = -P__.
If we suppose the radii of the spheres to become infinite, the case
becomes that of a point placed between two parallel planes A and H.
In this case these expressions become
T)~D
charge on A = —P — — ,
charge on £ = — P -— .
172.] In order to pass from this case to that of any two spheres
not intersecting each
other, we begin by
finding the two com
mon inverse points 0,
(7 through which all
circles pass that are
orthogonal to both
spheres. Then, invert
ing the system with
respect to either of
these points, the spheres
become concentric, as
rig. 15.
in the first case.
The radius OAPB on which the successive images lie becomes
an arc of a circle through 0 and (7, and the ratio of (/P to OP is
214 ELECTRIC IMAGES. [r72-
equal to Cen where C is a numerical quantity which for simplicity
we may make equal to unity.
We therefore put
... -or . VA _ , as
Let /3 — a = <*, u — a = 0.
Then all the successive images of P will lie on the arc OAPB&.
The position of the image of P in A is Q0 where
(70
^(«oM°g^| = 2a-^
That of Qo in £ is Px where
Similarly
u(P6) =
In the same way if the successive images of P in B, A, B, &c.
are Q0', P/, «/, &c.,
To find the charge of any image Ps we observe that in the
inverted figure its charge is
OP
In the original figure we must multiply this by (XP8. Hence the
charge of Ps in the dipolar figure is
A /°P*'
V "OP:
If we make f = VOP.(/P3 and call £ the parameter of the
point P, then we may write
or the charge of any image is proportional to its parameter.
If we make use of the curvilinear coordinates u and vt such that
x+ V -
Jc sin hu ksvbv m
coshu—cosv' ~~ cosku— -cosv 9
172.] TWO SPHERES NOT INTERSECTING. 215
x2 + (y— fccotv)2 = kz cosec2v,
(x + k cot huf + f = & cosec h?u,
\/COS /& W — COS V
Since the charge of each image is proportional to its parameter,
£, and is to be taken positively or negatively according as it is of
the form P or Q, we find
P\/coshu— cost;
JTt = — >
V cos ^ (u + 2 5 -BJ) — cos v
PVcoshu — cosv
V cos 7^(2 a — w — 2*-sr) — cos v
Pvcoshu — cos v
Vcos A(u — 2siv) — cos v
PV cos hit — cosv
Vcosk(2fi — u + 2s'&) — cosv
We have now obtained the positions and charges of the two
infinite series of images. We have next to determine the total
charge on the sphere A by finding the sum of all the images within
it which are of the form Q or P/. We may write this
— COSV
<*»m\ / z, n ,
1 Vcosn(u— 231*) — cosv
. _ *==°
— PVcoshu — cosv
-° //cos ^(2 a— u — 2 six) — cosv
In the same way the total induced charge on B is
— COS V *t-
cosv
— COSV
t-n I i i n ^
'-° Vcos/i(2j3 — u + 2s>&) — cosv
* In these expressions we must remember that
and the other functions of u are derived from these by the same definitions as the
corresponding trigonometrical functions.
The method of applying dipolar coordinates to this case was given by Thomson in
Lioui'ille's Journal for 1847. See Thomson's reprint of Electrical Papers, § 211, 212.
In the text I have made use of the investigation of Prof. Betti, Nuovo Cimento,
vol. xx, for the analytical method, but I have retained the idea of electrical images as
used by Thomson in his original investigation, Phil. Mag., 1853.
216 ELECTEIC IMAGES. [T73-
173.] We shall apply these results to the determination of the
coefficients of capacity and induction of two spheres whose radii are
a and #, and the distance of whose centres is <?.
In this case
sin/$.a=— > sin/£./3=T'
a b
Let the sphere A be at potential unity, and the sphere B at
potential zero.
Then the successive images of a charge a placed at the centre
of the sphere A will be those of the actual distribution of electricity.
All the images will lie on the axis between the poles and the
centres of the spheres.
The values of u and v for the centre of the sphere A are
u — 2 a, v = 0.
Hence we must substitute a or k - — j— for P. and 2 a for ut and
sin fia
v = 0 in the equations, remembering that P itself forms part of the
charge of A. We thus find for the coefficient of capacity of A
=o sn aw -a
for the coefficient of induction of A on B or of B on A
and for the coefficient of capacity of B
sn
To calculate these quantities in terms of a and #, the radii of the
spheres, and of c the distance between their centres, we make use
of the following quantities
TWO ELECTRIFIED SPHERES. 217
We may now write the hyperbolic sines in terms of p^ q, r ; thus
2S = OC
.=1
Proceeding1 to the actual calculation we find, either by this
process or by the direct calculation of the successive images as
shewn in Sir W. Thomson's paper, which is more convenient for
the earlier part of the series,
-
o
"
CLO U/~U~ u, u o
ab2 _ _ &
- f + "*
174.] We have then the following equations to determine the
charges Ea and Eb of the two spheres when electrified to potentials
7a and 7b respectively,
If We put qaaQbb — qab2 = & = ff >
and paa = qbb I/, pab = — qab D ', Pvb — <1™. &>
whence Paalhb — Pai? = & >
then the equations to determine the potentials in terms of the
charges are =
^^, and^>b6 are the coefficients of potential.
The total energy of the system is, by Art. 85,
+ 2
218 ELECTRIC IMAGES. [!74-
The repulsion between the spheres is therefore, by Arts. 92, 93,
F= +2 +
where c is the distance between the centres of the spheres.
Of these two expressions for the repulsion, the first, which
expresses it in terms of the potentials of the spheres and the
variations of the coefficients of capacity and induction, is the most
convenient for calculation.
We have therefore to differentiate the q's with respect to c.
These quantities are expressed as functions of k, a, /3, and -57, and
must be differentiated on the supposition that a and b are constant.
From the equations
. 7 j . , n sin h a sin h 8
K = a sin h a = b sm hfi = c : — j — »
„ , da sin h a cos hQ
we find -y- = — j—. — 7 >
dc k sin li TX
dp _ cos k a sin h /3
dc ~
d'ST 1
dk cos Ji a cos
dc sin h -ar
whence we find
dqaa cos h a cos h$ qaa s=&(sc — a cos h (3) cos h (s TV — a)
dc sin/fc'ar k ^*s=o c (sin^(«s-s7— a));
dqab ___ cos h a cos h ft qab
d<tbb _
c?<? sin h & k *s=o c (sin h ({3 + *-BT)) 2
Sir William Thomson has calculated the force between two
spheres of equal radius separated by any distance less than the
diameter of one of them. For greater distances it is not necessary
to use more than two or three of the successive images.
The series for the differential coefficients of the ^s with respect
to c are easily obtained by direct differentiation
1 7 5.] TWO SPHERES IN CONTACT. 219
dqaa _ 2a2bc 2a*b2c(2c2 -2b2-a2)
7/7 : - ^2-Zp2 ~ ~c*-* ~ "
__ab a2 b2 (3 c2 - a2 - b2)
"~~
a*b*{(5c2-a2-b2)(c2-a2-b2)-a2b2}
C2(c2-a2 -b2 + ab)2 (c2 -a2-b2- ab)2
clqbb _ 2ab2c 2a2b*c(2c2-2a2-b2)
~ ' ~ ~ ~ G'
Distribution of Electricity on Tivo Spheres in Contact.
175.] If we suppose the two spheres at potential unity and not
influenced by any other point, then, if we invert the system with
respect to the point of contact, we shall have two parallel planes,
distant — and —7 from the point of inversion, and electrified by
the action of a unit of electricity at that point.
There will be a series of positive images, each equal to unity, at
distances s( — (- j\ from the origin, where s may have any integer
value from — oo to -foo.
There will also be a series of negative images each equal to — 1,
the distances of which from the origin, reckoned in the direction of
1 A K
a, are - +s(- + 7)-
a \a b'
When this system is inverted back again into the form of the
two spheres in contact, we have a corresponding series of negative
images, the distances of which from the point of contact are of the
form — • - — , where s is positive for the sphere A and negative
/I 1\
*(-a + d
for the sphere B. The charge of each image, when the potential
of the spheres is unity, is numerically equal to its distance from the
point of contact, and is always negative.
There will also be a series of positive images whose distances
from the point of contact measured in the direction of the centre
of a, are of the form - - •
When s is zero, or a positive integer, the image is in the sphere A.
When s is a negative integer the image is in the sphere S.
220 ELECTEIC IMAGES. [175>
The charge of each image is measured by its distance from the
origin and is always positive.
The total charge of the sphere A is therefore
«=oo i ab
=
Each of these series is infinite, but if we combine them in the form
-6*=i s(a + b)(s(a+b}-d)
the series becomes converging.
In the same way we find for the charge of the sphere B}
^*=oo ab ab
•tth = 7, . —
The values of Ea and Ub are not^ so far as I know, expressible
in terms of known functions. Their difference, however,, is easily
expressed, for
i:ab
cot
a + b a-\-b
When the spheres are equal the charge of each for potential unity
s
= «loge2 = 1.0986^.
When the sphere A is very small compared with the sphere B
the charge on A is
E* = °j %I^ approximately;
^ a2
or Ea = --r
The charge on B is nearly the same as if A were removed, or
Eb = b.
The mean density on each sphere is found by dividing the charge
by the surface. In this way we get
1 77.] SPHERICAL BOWL. 221
E
Hence, if a very small sphere is made to touch a very large one,
the mean density on the small sphere is equal to that on the large
o
sphere multiplied by — , or 1.644936.
Application of Electrical Inversion to the case of a Spherical Bowl.
176.] One of the most remarkable illustrations of the power of
Sir W. Thomson's method of Electrical Images is furnished by his
investigation of the distribution of electricity on a portion of a
spherical surface bounded by a small circle. The results of this
investigation, without proof, were communicated to M. Liouville
and published in his Journal in 1847. The complete investigation
is given in the reprint of Thomson's Electrical Papers, Article XV.
I am not aware that a solution of the problem of the distribution
of electricity on a finite portion of any curved surface has been
given by any other mathematician.
As I wish to explain the method rather than to verify the
calculation, I shall not enter at length into either the geometry
or the integration, but refer my readers to Thomson's work.
Distribution of Electricity on an Ellipsoid.
177.] It is shewn by a well-known method* that the attraction
of a shell bounded by two similar and similarly situated and
concentric ellipsoids is such that there is no resultant attraction
on any point within the shell. If we suppose the thickness of
the shell to diminish indefinitely while its density increases, we
ultimately arrive at the conception of a surface-density varying
as the perpendicular from the centre on the tangent plane, and
since the resultant attraction of this superficial distribution on any
point within the ellipsoid is zero, electricity, if so distributed on
the surface,, will be in equilibrium.
Hence, the surface-density at any point of an ellipsoid undis
turbed by external influence varies as the distance of the tangent
plane from the centre.
* Thomson and Tait's Natural Philosophy, § 520, or Art. 150 of this book.
222 ELECTRIC IMAGES.
Distribution of Electricity on a Disk.
By making two of the axes of the ellipsoid equal, and making
the third vanish, we arrive at the case of a circular disk, and at an
expression for the surface-density at any point P of such a disk
when electrified to the potential V and left undisturbed by external
influence. If <r be the surface- density on one side of the disk,
and if KPL be a chord drawn through the point P, then
V
Application of the Principle of Electric Inversion.
178.] Take any point Q as the centre of inversion, and let R
be the radius of the sphere of inversion. Then the plane of the
disk becomes a spherical surface passing through Q, and the disk
itself becomes a portion of the spherical surface bounded by a circle.
We shall call this portion of the surface the bowl.
If S' is the disk electrified to potential V and free from external
influence, then its electrical image S will be a spherical segment at
potential zero, and electrified by the influence of a quantity Y'R of
electricity placed at Q.
We have therefore by the process of inversion obtained the
solution of the problem of the distribution of electricity on a
bowl or a plane disk when under the influence of an electrified
point in the surface of the sphere or plane produced.
Influence of an Electrified Point placed on the unoccupied part of the
Spherical Surface.
The form of the solution, as deduced by the principles already
given and by the geometry of inversion, is as follows :
If C is the central point or pole of the spherical bowl S, and
if a is the distance from C to any point in the edge of the segment,
then, if a quantity q of electricity is placed at a point Q in the
surface of the sphere produced, and if the bowl S is maintained
at potential zero, the density a- at any point P of the bowl will be
~ 27T2 QP2 A/ a2 -OP2'
CQ, CP, and QP being the straight lines joining the points, <?, Q,
and P.
It is remarkable that this expression is independent of the radius
of the spherical surface of which the bowl is a part. It is therefore
applicable without alteration to the case of a plane disk.
l8o.] SPHERICAL BOWL. 223
Influence of any Number of Electrified Points.
Now let us consider the sphere as divided into two parts, one of
which, the spherical segment on which we have determined the
electric distribution, we shall call the bowl, and the other the
remainder, or unoccupied part of the sphere on which the in
fluencing- point Q is placed.
If any number of influencing points are placed on the remainder
of the sphere, the electricity induced by these on any point of the
bowl may be obtained by the summation of the densities induced
by each separately.
179.] Let the whole of the remaining surface of the sphere
be uniformly electrified, the surface-density being p, then the
density at any point of the bowl may be obtained by ordinary
integration over the surface thus electrified.
We shall thus obtain the solution of the case in which the bowl
is at potential zero, and electrified by the influence of the remaining
portion of the spherical surface rigidly electrified with density p.
Now let the whole system be insulated and placed within a
sphere of diameter f, and let this sphere be uniformly and rigidly
electrified so that its surface-density is //.
There will be no resultant force within this sphere, and therefore
the distribution of electricity on the bowl will be unaltered, but
the potential of all points within the sphere will be increased by
a quantity V where
Hence the potential at every point of the bowl will now be V.
Now let us suppose that this sphere is concentric with the sphere
of which the bowl forms a part, and that its radius exceeds that
of the latter sphere by an infinitely small quantity.
We have now the case of the bowl maintained at potential V and
influenced by the remainder of the sphere rigidly electrified with
superficial density p -f p'.
180.] We have now only to suppose p-j-p'= 0, and we get the
case of the bowl maintained at potential V and free from external
influence.
If <T is the density on either surface of the bowl at a given point
when the bowl is at potential zero, and is influenced by the rest
of the sphere electrified to density p, then, when the bowl is main
tained at potential F, we must increase the density on the outside
of the bowl by p', the density on the supposed enveloping sphere.
224
ELECTRIC IMAGES.
[181.
The result of this investigation is that if f is the diameter of
the sphere, a the chord of the radius of the bowl, and r the chord
of the distance of P from the pole of the bowl, then the surface-
density <r on the inside of the bowl is
27r2/
and the surface-density on the outside of the bowl at the same
point is V
+
27T/
In the calculation of this result no operation is employed more
abstruse than ordinary integration over part of a spherical surface.
To complete the theory of the electrification of a spherical bowl
we only require the geometry of the inversion of spherical surfaces.
181.] Let it be required to find the surface-density induced at
any point of the bowl by a quantity q of electricity placed at a
point Q, not now in the spherical surface produced.
Invert the bowl with respect to Q, the radius of the sphere of
inversion being R. The bowl 8 will be inverted into its image $",
and the point P will have P for its image. We have now to
determine the density </ at P' when the bowl 8' is maintained at
potential V ', such that q = V'R, and is not influenced by any
external force.
The density o- at the point P of the original bowl is then
<J = —
QP'1
this bowl being at potential zero, and influenced by a quantity q of
electricity placed at Q.
The result of this process is as follows :
Let the figure represent a section
through the centre, 0, of the sphere,
the pole, C} of the bowl, and the in
fluencing point Q. D is a point
which corresponds in the inverted
figure to the unoccupied pole of the
rim of the bowl, and may be found
by the following construction.
Draw through Q the chords EQE'
and FQF', then if we suppose the
radius of the sphere of inversion to
be a mean proportional between the
segments into which a chord is divided at Q, E'F' will be the
Fig. 16.
1 8 1.] SPHERICAL BOWL. 225
image of EF. Bisect the arc F'CE' in I/, so that F' &=!/&, and
draw J/QD to meet the sphere in D. D is the point required.
Also through 0, the centre of the sphere, and Q draw HOQH.'
meeting the sphere in H and H'. Then if P be any point in the
bowl, the surface-density at P on the side which is separated from
Q by the completed spherical surface, induced by a quantity q of
electricity at Q, will be
~
where a denotes the chord drawn from <?, the pole of the bowl,
to the rim of the bowl.
On the side next to Q the surface-density is
q QH.QH'
f 27T2 HH'.PQ*'
VOL. I.
CHAPTER XII.
THEORY OF CONJUGATE FUNCTIONS IN TWO DIMENSIONS.
182.] THE number of independent cases in which the problem
of electrical equilibrium has been solved is very small. The method
of spherical harmonics has been employed for spherical conductors,
and the methods of electrical images and of inversion are still more
powerful in the cases to which they can be applied. The case of
surfaces of the second degree is the only one, as far as I know,
in which both the equipotential surfaces and the lines of force are
known when the lines of force are not plane curves.
But there is an important class of problems in the theory of
electrical equilibrium, and in that of the conduction of currents,
in which we have to consider space of two dimensions only.
For instance, if throughout the part of the electric field under
consideration, and for a considerable distance beyond it, the surfaces
of all the conductors are generated by the motion of straight lines
parallel to the axis of z, and if the part of the field where this
ceases to be the case is so far from the part considered that the
electrical action of the distant part on the field may be neglected,
then the electricity will be uniformly distributed along each gene
rating line, and if we consider a part of the field bounded by two
planes perpendicular to the axis of z and at distance unity, the
potential and the distribution of electricity will be functions of x
and y only.
If p dx dy denotes the quantity of electricity in an element whose
base is dx dy and height unity, and a- ds the quantity on an element
of area whose base is the linear element ds and height unity, then
the equation of Poisson may be written
183.] PROBLEMS IN TWO DIMENSIONS. 227
When there is no free electricity, this is reduced to the equation
of Laplace,
The general problem of electric equilibrium may be stated as
follows : —
A continuous space of two dimensions, bounded by closed curves
C19 C2, &c. being given, to find the form of a function, F, such that
at these boundaries its value may be F15 F2, &c. respectively, being
constant for each boundary, and that within this space V may be
everywhere finite, continuous, and single valued, and may satisfy
Laplace's equation.
I am not aware that any perfectly general solution of even this
question has been given, but the method of transformation given in
Art. 190 is applicable to this case, and is much more powerful than
any known method applicable to tbree dimensions.
The method depends on the properties of conjugate functions of
two variables.
Definition of Conjugate Functions.
183.] Two quantities a and /3 are said to be conjugate functions
of x and y^ if a + V— T /3 is a function of x -f \/ — 1 y.
It follows from this definition that
do. d(3 da d8
=' and + = °; w
dx2 dy* ~ dz2 , dy* '
Hence both functions satisfy Laplace's equation. Also
dadft__dadft_d^2 Jo* _~dfi2 ^2_«2
dx dy dy dx ~ dx\ + dy\ ~" dx h ~fy\ ~~
If x and y are rectangular coordinates, and if ds^ is the intercept
of the curve (/3 = constant) between the curves a and a -f da, and
ds.2 the intercept of a between the curves /3 and /3 -f ^/3, then
ds,_d^_ 1
da ~ dp " R
and the curves intersect at right angles.
If we suppose the potential F = F0-f,£a, where k is some con
stant, then V will satisfy Laplace's equation, and the curves (a) will
be equipotential curves. The curves (/3) will be lines of force, and
228 CONJUGATE FUNCTIONS. [184.
the surface-integral of a surface whose projection on the plane of
xy is the curve AB will be k(pB—pA\ where $A and (3B are the
values of (3 at the extremities of the curve.
If a series of curves corresponding1 to values of a in arithmetical
progression is drawn on the plane, and another series corresponding
to a series of values of /3 having the same common difference, then
the two series of curves will everywhere intersect at right angles,
and, if the common difference is small enough, the elements into
which the plane is divided will be ultimately little squares, whose
sides, in different parts of the field, are in different directions and of
different magnitude, being inversely proportional to R.
If two or more of the equipotential lines (a) are closed curves
enclosing a continuous space between them, we may take these for
the surfaces of conductors at potentials (VQ + ka^, (F"04-^a2), &c.
respectively. The quantity of electricity upon any one of these
Jc
between the lines of force ^ and /32 will be — Oa— &)•
The number of equipotential lines between two conductors will
therefore indicate their difference of potential, and the number of
lines of force which emerge from a conductor will indicate the
quantity of electricity upon it.
We must next state some of the most important theorems
relating to conjugate functions, and in proving them we may use
either the equations (1), containing the differential coefficients, or
the original definition, which makes use of imaginary symbols.
184.] THEOREM I. If x' and y' are conjugate functions with respect
to x and y, and if x" and y" are also conjugate functions with
respect to x and y, then the functions x' + x" and y' +y" will
le conjugate functions with respect to x
dx' _dy' dx" _dy" .
~7~ - ~T~ 5 <*-U.U — ~ - = — - - ,
dx ay dx dy
therefore
dx dy
A1 dx' dy' dx" dy"
Also =_^L and -— = - J— ;
dy dx dy dx
therefore
dy dx
or x -fa?" and /+/' are conjugate with respect to x and y.
185.] GRAPHIC METHOD. 229
Graphic Representation of a Function which is the Sum of Two
Given Functions.
Let a function (a) of x and y be graphically represented by a
series of curves in the plane of xy, each of these curves corre
sponding to a value of a which belongs to a series of such values
increasing by a common difference, b.
Let any other function, (3, of x and y be represented in the same
way by a series of curves corresponding to a series of values of j3
having the same common difference as those of a.
Then to represent the function a + ft in the same way, we must
draw a series of curves through the intersections of the two former
series from the intersection of the curves (a) and (/3) to that of the
curves (a +6) and (/3 — 8), then through the intersection of (a 4- 2 b)
and (/3 — 2 b), and so on. At each of these points the function will
have the same value, namely a + ft. The next curve must be drawn
through the points of intersection of a and /3 + 5, of a + b and ft,
of a + 2 b and /3 — 8, and so on. The function belonging to this
curve will be a + /3 + 8.
In this way, when the series of curves (a) and the series (/3) are
drawn, the series (a + (3) may be constructed. These three series of
curves may be drawn on separate pieces of transparent paper, and
when the first and second have been properly superposed, the third
may be drawn.
The combination of conjugate functions by addition in this way
enables us to draw figures of many interesting cases with very
little trouble when we know how to draw the simpler cases of
which they are compounded. We have, however, a far more
powerful method of transformation of solutions, depending on the
following theorem.
185.] THEOREM II. If x" and y" are conjugate functions with
respect to the variables x' and y' , and if x' and y' are conjugate
functions with respect to x and y> then x" and y" will be con
jugate functions with respect to x and y.
dx" dx" dx' dx" dy'
For — T- = -7-7- -5- + -^-7- -y- >
dx dx dx dy dx
df dy' dy" dx'
dy' dy dx' dy '
*r_.
dy'
230 CONJUGATE FUNCTIONS.
dx" dx" dx' dx" dy'
and — — = -— r — - + -j— -f- ,
dy dx dy dy dy
dy" dy' dy" dx'
dy' dx dx' dx
' dx '
and these are the conditions that x" and y" should be conjugate
functions of x and y.
This may also be shewn from the original definition of conjugate
functions. For #"-f\/^iy is a function of xf + V — 1 y ', and
x' + \/ — iy is a function of # + \/— ly. Hence, x" + \/ — \y"
is a function of # + \/— \y.
In the same way we may shew that if af and y' are conjugate
functions of x and yy then x and y are conjugate functions of x'
and y.
This theorem may be interpreted graphically as follows : —
Let x' , y' be taken as rectangular coordinates, and let the curves
corresponding to values of x" and of y" taken in regular arithmetical
series be drawn on paper. A double system of curves will thus be
drawn cutting the paper into little squares. Let the paper be also
ruled with horizontal and vertical lines at equal intervals, and let
these lines be marked with the corresponding values of x' and y' .
Next, let another piece of paper be taken in which x and y are
made rectangular coordinates and a double system of curves of, y
is drawn, each curve being marked with the corresponding value
of x' or y' . This system of curvilinear coordinates will correspond,
point for point, to the rectilinear system of coordinates x' ', y' on the
first piece of paper.
Hence, if we take any number of points on the curve x" on the
first paper, and note the values of x' and y' at these points, and
mark the corresponding points on the second paper, we shall find
a number of points on the transformed curve x" ' . If we do the
same for all the curves x" ', y" on the first paper, we shall obtain on
the second paper a double series of curves of' ', y" of a different form,
but having the same property of cutting the paper into little
squares.
1 86.]
THEOREMS.
231
186.] THEOREM III. If V is any function of V and y' ', and if of
and if are conjugate functions of oo and y, then
integration being between the same limits.
dV dVdx' dYdif
-j- = -j-> -J- + -T-* -j- '
«a? a* 00 dy dx
^_
dxz ~ dx'*
dx' dy'
"h dx'd' dx dx + d'2 dx
and
dx'
dy\ dx'dy' dy dy
Adding the last two equations, and remembering the conditions
of conjugate functions (1), we find
_
~ dx72
_ ,d2Y
~ W2
2 "7~?2
dx ;
Hence
dy'
dx dy dy dx
If V is a potential, then, by Poisson's equation
and we may write the result
ffpdxdy=ffp'<lx'dy',
or the quantity of electricity in corresponding portions of two
systems is the same if the coordinates of one system are conjugate
functions of those of the other.
232 CONJUGATE FUNCTIONS. [187.
Additional Theorems on Conjugate Functions.
187.] THEOREM IV. If x^ and yl3 and also #2 and y^ are con
jugate functions of x and y, then, if
X=xlLxz-M2, and r= ^^ + 3^,
X and T will be conjugate functions of x and y.
For X+ y^Tr = + V
THEOREM V. If </> be a solution of the equation
^ ^0
dx* + df ' '
and e =
0 will be conjugate functions of x and y.
For R and 0 are conjugate functions of —7^ and -~- , and these
, r , . J r& , dx dy
are conjugate functions of x and y.
EXAMPLE I. — Inversion.
188.] As an example of the general method of transformation
let us take the case of inversion in two dimensions.
If 0 is a fixed point in a plane, and OA a fixed direction, and
if r = OP = ae?y and 0 = AOP, and if x, y are the rectangular
coordinates of P with respect to 0,
p and Q are conjugate functions of x and y.
If // = np and 0' = nO, p' and ^ will be conjugate functions of p
and 0. In the case in which n = — 1 we have
/ = — , and B'=-e, (6)
which is the case of ordinary inversion combined with turning the
figure 180° round OA.
Inversion in Two Dimensions.
In this case if r and / represent the distances of corresponding
i89-]
ELECTRIC IMAGES IN TWO DIMENSIONS.
233
points from 0, e and / the total electrification of a body, 8 and &'
superficial elements, V and V solid elements, a- and </ surface-
densities, p and p volume densities, $ and <f>' corresponding po
tentials,
a* /2
~s
EL
~r
(7)
EXAMPLE II. — Electric Images in Two Dimensions.
189.] Let A be the centre of a circle of radius AQ = b, and let
E be a charge at A3 then the potential
at any point P is
b Q.'
and if the circle is a section of a hollow
conducting cylinder, the surface-density
E
at any point Q is — -—7
Fig. 17.
Invert the system with respect to a point 0, making
AO = mb, and a2 = (m2-l)b2 ;
then we have a charge at A equal to that at A} where AA' = — •
The density at Q' is
E W-ZIf
~2ri> A'Q'2
and the potential at any point Pf within the circle is
<£' = 0 = 2 E (log b— log AP),
= 2^ (log 0P'-log^'P'-log»). (9)
This is equivalent to a combination of a charge ^ at ^', and a
charge — J£ at 0, which is the image of A', with respect to the
circle. The imaginary charge at 0 is equal and opposite to that
If the point P' is defined by its polar coordinates referred to the
centre of the circle, and if we put
p = log r— log b, and p0 = log AA' — log 5,
then AP = be?. AA'^be?*, A0 = be-p*', (10)
234 CONJUGATE FUNCTIONS. [190.
and the potential at the point (p, 6} is
— E log (e2Po — 2 eK e? cos 0 + e2?) -f 2 Ep0. (11)
This is the potential at the point (p, 0) due to a charge E, placed
at the point (p0, 0), with the condition that when p = 0, <p = 0.
In this case p and 0 are the conjugate functions in equations (5) :
p is the logarithm of the ratio of the radius vector of a point to
the radius of the circle, and 6 is an angle.
The centre is the only singular point in this system of coordinates,
and the line-integral of / -=- ds round a closed curve is zero or 2 TT,
J U/S
according as the closed curve excludes or includes the centre.
EXAMPLE III. — Neumann's Transformation of this Case *.
190.] Now let a and ft be any conjugate functions of x and y,
such that the curves (a) are equipotential curves, and the curves
(ft) are lines of force due to a system consisting of a charge of half
a unit at the origin, and an electrified system disposed in any
manner at a certain distance from the origin.
. Let us suppose that the curve for which the potential is a0 is
a closed curve, such that no part of the electrified system except the
half-unit at the origin lies within this curve.
Then all the curves (a) between this curve and the origin will be
closed curves surrounding the origin, and all the curves (ft) will
meet in the origin, and will cut the curves (a) orthogonally.
The coordinates of any point within the curve (a0) will be determ
ined by the values of a and ft at that point, and if the point travels
round one of the curves a in the positive direction, the value of ft
will increase by 2 TT for each complete circuit.
If we now suppose the curve (a0) to be the section of the inner
surface of a hollow cylinder of any form maintained at potential
zero under the influence of a charge of linear density E on a line of
which the origin is the projection, then we may leave the external
electrified system out of consideration, and we have for the potential
at any point (a) within the curve
<*> = 2^(a-a0), (12)
and for the quantity of electricity on any part of the curve a0
between the points corresponding to ft1 and ftz,
q = 2J0G8.-/9,). (13)
« See Crelie's Journal, 1861.
190.] NEUMANN'S TRANSFORMATION. 235
If in this way, or in any other, we have determined the dis
tribution of potential for the case of a given curve of section when
the charge is placed at a given point taken as origin, we may pass
to the case in which the charge is placed at any other point by an
application of the general method of transformation.
Let the values of a and /3 for the point at which the charge is
placed be ax and ft, then substituting in equation (11) a— ct0 for p,
and ft— ft for 6, we find for the potential at any point whose co
ordinates are a and ft
$ = Elog (l-2ea-«icos((3-l31) + e2(a-aJ)
-.E'log (l-2^+ai-2aocos(/3-^1) + ^2(a+ai-2ao)) + 2^(a1-a0). (14)
This expression for the potential becomes zero when a=a0, and is
finite and continuous within the curve a0 except at the point at ft ,
at which point the first term becomes infinite, and in its immediate
neighbourhood is ultimately equal to 2E log/, where / is the
distance from that point.
We have therefore obtained the means of deducing the solution
of Green's problem for a charge at any point within a closed curve
when the solution for a charge at any other point is known.
The charge induced upon an element of the curve a0 between the
points /3 and /3 + dp by a charge E placed at the point a: ft is
_ _ - _ JB (l ~
27T l-2^al-ao)cOS(/3-ft) + *2(al-«o)
From this expression we may find the potential at any point
ai A within the closed curve, when the value of the potential at
every point of the closed curve is given as a function of j3, and
there is no electrification within the closed curve.
For, by Theorem II of Chap. Ill, the part of the potential at
ax ft, due to the maintenance of the portion dj3 of the closed curve
at the potential F, is n V, where n is the charge induced on df$ by
unit of electrification at c^ft. Hence, if V is the potential at a
point on the closed curve defined as a function of ft and $ the
potential at the point a, ft within the closed curve, there being no
electrification within the curve,
~ (16)
-
— 2e(»i-a<i>cos(/3— j
236 CONJUGATE FUNCTIONS. [I9i-
EXAMPLE IV. — Distribution of Electricity near an Edge of a
Conductor formed by Two Plane Faces.
191.] In the case of an infinite plane face of a conductor charged
with electricity to the surface -density o-0, we find for the potential
at a distance y from the plane
where C is the value of the potential of the conductor itself.
Assume a straight line in the plane as a polar axis, and transform
into polar coordinates, and we find for the potential
V= C— 4 77 o-0 # £P sin 0,
and for the quantity of electricity on a parallelogram of breadth
unity, and length ae? measured from the axis
E = (T^ae?.
Now let us make p = np and 0 = n6', then, since p and 0' are
conjugate to p and 0, the equations
V = (7—4 77 o-0 aen?' sin nO'
and E = v^ae1"?'
express a possible distribution of electricity and of potential.
If we write r for ae? y r will be the distance from the axis, and
6 the angle, and we shall have
y.n
V = C — 4TTO-Q n, sin n 6,
a
V will be equal to C whenever nd = 77 or a multiple of 77.
Let the edge be a salient angle of the conductor, the inclination
of the faces being a, then the angle of the dielectric is 2 IT— a, so
that when 6 = 2 TT — a the point is in the other face of the conductor.
We must therefore make
n(2it— a) = 77,
Then r=C-4ir<r0a(-\~ "sin ^
° W 277 —
The surface-density <r at any distance r from the edge is
a — TT
dE -ir ,r**-*
1Q2.] ELLIPSES AND HYPERBOLAS. 237
When the angle is a salient one a is less than 77, and the surface-
density varies according- to some inverse power of the distance
from the edge, so that at the edge itself the density becomes
infinite, although the whole charge reckoned from the edge to any
finite distance from it is always finite.
Thus, when a = 0 the edge is infinitely sharp, like the edge of a
mathematical plane. In this case the density varies inversely as
the square root of the distance from the edge.
When a = — the edge is like that of an equilateral prism, and the
o
density varies inversely as the ^ power of the distance.
When a = - the edge is a right angle, and the density is in-
2
versely as the cube root of the distance.
o _
When a = — - the edge is like that of a regular hexagonal prism,
o
and the density is inversely as the fourth root of the distance.
When a = 77 the edge is obliterated, and the density is constant.
When a = -f TT the edge is like that in the inside of the hexagonal
prism, and the density is directly as the square root of the distance
from the edge.
When a=f 77 the edge is a re-entrant right angle, and the density
is directly as the distance from the edge.
When a=-f 77 the edge is a re-entrant angle of 60°, and the
density is directly as the square of the distance from the edge.
In reality, in all cases in which the density becomes infinite at
any point, there is a discharge of electricity into the dielectric at
that point, as is explained in Art. 55.
EXAMPLE V. — Ellipses and Hyperbolas. Fig. X.
192.] We have seen that if
#j_ = e* cos ty, y^ = e* sin x//-, (1)
x and y will be conjugate functions of $ and ^.
Also, if x2 = er* cos \j/, y% = —e~* sin \^, (2)
x.2 and y2 will be conjugate functions. Hence, if
2 a? = ^ + ^2= («* + <?-*) cos VT, 2y = ft + #j = (<?*-*-*) sin ijr, (3)
x and y will also be conjugate functions of $ and \f/.
In this case the points for which $ is constant lie in the ellipse
whose axes are & -f er* and e* — er*.
238 CONJUGATE FUNCTIONS.
The points for which \l/ is constant lie in the hyperbola whose
axes are 2 cos \j/ and 2 sin \//.
On the axis of x, between x = — I and x = + I,
<f) = 0, \js = cos-1 x. (4)
On the axis of #, beyond these limits on either side, we have
x> 1, $ = 0, <£ = log (# + y^T), (5)
Hence,, if <p is the potential function, and \j/ the function of flow,
we have the case of electricity flowing- from the negative to the
positive side of the axis of x through the space between the points
— 1 and + 1 , the parts of the axis beyond these limits being
impervious to electricity.
Since, in this case, the axis of y is a line of flow, we may suppose
it also impervious to electricity.
We may also consider the ellipses to be sections of the equi-
potential surfaces due to an indefinitely long flat conductor of
breadth 2, charged with half a unit of electricity per unit of length.
If we make \ff the potential function, and <£ the function of flow,
the case becomes that of an infinite plane from which a strip of
breadth 2 has been cut away and the plane on one side charged to
potential IT while the other remains at zero.
These cases may be considered as particular cases of the quadric
surfaces treated of in Chapter X. The forms of the curves are
given in Fig. X.
EXAMPLE VI.— Fig. XI.
193.] Let us next consider oc' and yf as functions of x and y, where
^!/
x'— b log V#2-f y2, y' = I tan-1 - » (6)
af and y' will be also conjugate functions of $ and \/r.
The curves resulting from the transformation of Fig. X with
respect to these new coordinates are given in Fig. XI.
If x and y' are rectangular coordinates, then the properties of the
axis of x in the first figure will belong to a series of lines parallel
to x' in the second figure for which y' — bn'ir, where n' is any
integer.
The positive values of xf on these lines will correspond to values
of x greater than unity, for which, as we have already seen,
__ / £ /~2^ \
\l? = rnr, $ — log(>+ ^/X2—1) = log U& + V eb —I/. (7)
195 •] EDGE OF AN ELECTRIFIED PLATE. 239
The negative values of x' on the same lines will correspond to
values of x less than unity, for which, as we have seen,
£
<f> = 0, \l/ = cos~l x = cos-1 eb . (8)
The properties of the axis of y in the first figure will belong to
a series of lines in the second figure parallel to #', for which
/ = $*(»' + £). (9)
The value of \ff along these lines is \j/ = 77 (nr -f J-) for all points
both positive and negative, and
0 = log (y + vV + 0 = lo£ U* + V eb~ + I/. (10)
194.] If we consider <p as the potential function, and \/r as the
function of flow, we may consider the case to be that of an in
definitely long strip of metal of breadth it d with a non-conducting
division extending from the origin indefinitely in the positive
direction, and thus dividing the positive part of the strip into two
separate channels. We may suppose this division to be a narrow
slit in the sheet of metal.
If a current of electricity is made to flow along one of these
divisions and back again along the other, the entrance and exit of
the current being at an indefinite distance on the positive side of
the origin, the distribution of potential and of current will be given
by the functions $ and \/r respectively.
If, on the other hand, we make i/r the potential, and $ the
function of flow, then the case will be that of a current in the
general direction of y, flowing through a sheet in which a number
of non-conducting divisions are placed parallel to x, extending from
the axis of y to an indefinite distance in the negative direction.
195.] We may also apply the results to two important cases in
statical electricity.
(1) Let a conductor in the form of a plane sheet, bounded by a
straight edge but otherwise unlimited, be placed in the plane of xz
on the positive side of the origin, and let two infinite conducting
planes be placed parallel to it and at distances \-nb on either side.
Then, if \js is the potential function, its value is 0 for the middle
conductor and J TT for the two planes.
Let us consider the quantity of electricity on a part of the middle
conductor, extending to a distance 1 in the direction of z, and from
the origin to x = a.
The electricity on the part of this strip extending from xl to x.2
is -<-
240 CONJUGATE FUNCTIONS. [196.
Hence from the origin to x'= a the amount is
E= — log(eb + v eb — l) - (11)
47T
If a is large compared with &, this becomes
Hence the quantity of electricity on the plane hounded by the
straight edge is greater than it would have been if the electricity
had been uniformly distributed over it with the same density that
it has at a distance from the boundary, and it is equal to the
quantity of electricity having the same uniform surface-density,
but extending to a breadth equal to b loge 2 beyond the actual
boundary of the plate.
This imaginary uniform distribution is indicated by the dotted
straight lines in Fig. XI. The vertical lines represent lines of
force, and the horizontal lines equipotential surfaces, on the hypo
thesis that the density is uniform over both planes, produced to
infinity in all directions.
196.] Electrical condensers are sometimes formed of a plate
placed midway between two parallel plates extending considerably
beyond the intermediate one on all sides. If the radius of curvature
of the boundary of the intermediate plate is great compared with
the distance between the plates, we may treat the boundary as
approximately a straight line, and calculate the capacity of the
condenser by supposing the intermediate plate to have its area
extended by a strip of uniform breadth round its boundary, and
assuming the surface-density on the extended plate the same as
it is in the parts not near the boundary.
Thus, if 8 be the actual area of the plate, L its circumference,
and B the distance between the large plates, we have
b = -B, (13)
TT
and the breadth of the additional strip is
« = ^.-B, (14)
IT
so that the extended area is
8'= 8 + 3L-}o2. (15)
7T
196.] DENSITY NEAR THE EDGE. 241
The capacity of the middle plate is
Correction for the Thickness of the Plate.
Since the middle plate is generally of a thickness which cannot
be neglected in comparison with the distance between the plates,
we may obtain a better representation of the facts of the case by
supposing the section of the intermediate plate to correspond with
the curve -fy = \j/' '.
The plate will be of nearly uniform thickness, (3 = 2#\//, at a
distance from the boundary, but will be rounded near the edge.
The position of the actual edge of the plate is found by putting
/ = 0, whence xf — I log cos ^'. (17)
The value of (p at this edge is 0, and at a point for which x'= a
it is a + I loge 2
~~b~
Hence the quantity of electricity on the plate is the same as
if a strip of breadth JR. , w£v n ft>
a'= — loge(2cos^)
had been added to the plate, the density being assumed to be every
where the same as it is at a distance from the boundary.
Density near the Edge.
The surface-density at any point of the plate is
*
1 d$ _ 1 <?T
4 TT dxf ~ 477$ /~2*T~
V*6~_l
1 / _!£.' _4^ \
=aiVl^i« & + |^ 6 -&C.A (19)
The quantity within brackets rapidly approaches unity as x'
increases, so that at a distance from the boundary equal to n times
the breadth of the strip a, the actual density is greater than the
normal density by about 2n+1 of the normal density.
In like manner we may calculate the density on the infinite planes
(20)
When #' = 0, the density is 2~^ of the normal density.
VOL. i. R
242 CONJUGATE FUNCTIONS.
At n times the breadth of the strip on the positive side, the
density is less than the normal density by about +1 •
At n times the breadth of the strip on the negative side, the
density is about — of the normal density.
These results indicate the degree of accuracy to be expected in
applying this method to plates of limited extent, or in which
irregularities may exist not very far from the boundary. The same
distribution would exist in the case of an infinite series of similar
plates at equal distances, the potentials of these plates being
alternately + V and — V. In this case we must take the distance
between the plates equal to B.
197.] (2) The second case we shall consider is that of an infinite
series of planes parallel to x z at distances £ = lib, and all cut off by
the plane of yz> so that they extend only on the negative side of this
plane. If we make (/> the potential function, we may regard these
planes as conductors at potential zero.
Let us consider the curves for which ^> is constant.
When y' = nirb, that is, in the prolongation of each of the planes,
we have j = 6 log i ^ + e-^ ^
when y'= (n+\}bir, that is, in the intermediate positions
^=5 log i(«* -«-*). (22)
Hence, when <£ is large, the curve for which 0 is constant is
an undulating line whose mean distance from the axis of y is .
approximately
a = bfr-log.!), (23)
and the amplitude of the undulations on either side of this line is
When <p is large this becomes be~2^} so that the curve approaches
to the form of a straight line parallel to the axis of y at a distance
a from ab on the positive side.
If we suppose a plane for which of = a, kept at a constant
potential while the system of parallel planes is kept at a different
potential, then, since 6$ = a + b loge 2, the surface-density of
the electricity induced on the plane is equal to that which would
have been induced on it by a plane parallel to itself at a potential
equal to that of the series of planes, but at a distance greater
than that of the edges of the planes by b log 2.
1 99.] A GROOVED SURFACE. 243
If B is the distance between two of the planes of the series,
B = TT b, so that the additional distance is
a =5^. (25)
198.] Let us next consider the space included between two of
the equipotential surfaces, one of which consists of a series of parallel
waves, while the other corresponds to a large value of $, and may
be considered as approximately plane.
If D is the depth of these undulations from the crest to the trough
of each wave, then we find for the corresponding value of <£,
D
^. (26)
The value of of at the crest of the wave is
6 log i(e* + (?-*). (27)
Hence, if A is the distance from the crests of the waves to the
opposite plane, the capacity of the system composed of the plane
surface and the undulated surface is the same as that of two planes
at a distance A + a where
«' = vlos«— ^- (28)
7T — 7T -
l+e B
199.] If a single groove of this form be made in a conductor
having the rest of its surface plane, and if the other conductor is
a plane surface at a distance A, the capacity of the one conductor
with respect to the other will be diminished. The amount of this
diminution will be less than the -th part of the diminution due
n
to n such grooves side by side, for in the latter case the average
electrical force between the conductors will be less than in the
former case, so that the induction on the surface of each groove will
be diminished on account of the neighbouring grooves.
If L is the length, B the breadth, and D the depth of the groove,
the capacity of a portion of the opposite plane whose area is S will be
8 LB a'
4 TT A 4iK A.A + a
If A is large compared with B or a', the correction becomes
(29)
R 2
244: CONJUGATE FUNCTIONS. [2OO.
and for a slit of infinite depth, putting D = oo, the correction is
To find the surface-density on the series of parallel plates we
must find o- = — -f-f when d> = 0. We find
V e 2&- 1
The average density on the plane plate at distance A from the
edges of the series of plates is o- = — 7 . Hence, at a distance from
the edge of one of the plates equal to na the surface- density is
of this average density.
200.] Let us next attempt to deduce from these results the
distribution of electricity in the figure formed by rotating the
plane of the figure about the axis y'=—R. In this case, Poisson's
equation will assume the form
dV
°' (88)
Let us assume F^=<£, the function given in Art. 193, and determine
the value of p from this equation. We know that the first two
terms disappear, and therefore
" = -i^?7^- (34)
If we suppose that, in addition to the surface-density already
investigated, there is a distribution of electricity in space according
to the law just stated, the distribution of potential will be repre
sented by the curves in Fig. XI.
Now from this figure it is manifest that -=-7 is generally very
"
small except near the boundaries of the plates, so that the new
distribution may be approximately represented by what actually
exists, namely a certain superficial distribution near the edges of
the plates.
If therefore we integrate / I pdaicty between the limits ,/=0 and
y'=-5, and from #'=— oo to # = +oo, we shall find the whole
2i
additional charge on one side of the plates due to the curvature.
201.] THEORY OF THOMSONS GUARD-RING. 245
0. d(fr d\j/
Since -— = ^-, >
ay dx
Integrating with respect to y', we find
This is the total quantity of electricity which we must suppose
distributed in space near the positive side of one of the cylindric
plates per unit of circumference. Since it is only close to the edge
of the plate that the density is sensible, we may suppose it all
condensed on the surface of the plate without altering sensibly its
action on the opposed plane surface, and in calculating the attraction
between that surface and the cylindric surface we may suppose this
electricity to belong to the cylindric surface.
The superficial charge on the positive surface of the plate per
unit of length would have been — J, if there had been no curvature.
T)
Hence this charge must be multiplied by the factor (l +i~j»)
to get the total charge on the positive side.
In the case of a disk of radius R placed midway between two
infinite parallel plates at a distance £, we find for the capacity
of the disk T>,, } 0
*+S**!*S + l3. (38)
.£> 77
Theory of Thomson's Guard-ring.
201.] In some of Sir W. Thomson's electrometers, a large plane
surface is kept at one potential, and at a distance a from this surface
is placed a plane disk of radius R surrounded by a large plane plate
called a Guard-ring with a circular aperture of radius R concentric
with the disk. This disk and plate are kept at potential zero.
The interval between the disk and the guard-plate may be
regarded as a circular groove of infinite depth, and of breadth
K—R, which we denote by B.
246 CONJUGATE FUNCTIONS. \_2O2.
The charge on the disk due to unit potential of the large disk,
7?2
supposing the density uniform, would be --
4 ^L
The charge on one side of a straight groove of breadth B and
length It = 27T.Z2, and of infinite depth, would be
RB
But since the groove is not straight, but has a radius of curvature
j)
R, this must be multiplied by the factor (l + i -~-) •
The whole charge on the disk is therefore
8A SA
The value of a cannot be greater than
R>*-R* g'
'
, =0.22 B nearly. -,
If B is small compared with either A or R this expression will
give a sufficiently good approximation to the charge on the disk
due to unity of difference of potential. The ratio of A to R
may have any value, but the radii of the large disk and of the
guard-ring must exceed R by several multiples of A.
EXAMPLE VII.— Fig. XII.
202.] Helmholtz, in his memoir on discontinuous fluid motion *,
has pointed out the application of several formulae in which the
coordinates are expressed as functions of the potential and its
conjugate function.
One of these may be applied to the case of an electrified plate
of finite size placed parallel to an infinite plane surface connected
with the earth.
Since x^ = A <£ and y^ — A ty,
and also #2 = A e* cos \//- and y^ = A e^ sin \j/,
are conjugate functions of <£ and \fr, the functions formed by adding
#! to #2 and^ to y^ will be also conjugate. Hence, if
x = A$ + Ae$ cos \js,
y =• A \lr-\- A e$ sin if/,
* Konigl. AkacL der Wissenschaften, zu Berlin, April 23, 1868.
202.] TWO EQUAL DISKS. 247
then x and y will be conjugate with respect to $ and \\r, and <£ and
\j/ will be conjugate with respect to x and y.
Now let x and y be rectangular coordinates, and let k-ty be the
potential, then £(/> will be conjugate to k^, Jc being any constant.
Let us put \lf = TT, then y = ^TT, # = J (<£ — **).
If <£ varies from — oo to 0, and then from 0 to +00, x varies
from —oo to — A and from — A to — oo. Hence the equipotential
surface for which k^\f — TT is a plane parallel to # at a distance b = it A
from the origin, and extending from — oo to x = — A.
Let us consider a portion of this plane, extending from
x = — (A -f a) to x = —A and from z = 0 to z = c,
let us suppose its distance from the plane of xz to be y — b = ATT,
and its potential to be V = k ^ = Jc TT.
The charge of electricity on any portion of this part of the plane
is found by ascertaining the values of $ at its extremities.
If these are fa and fa, the quantity of electricity is
— ck(fa-fa).
We have therefore to determine <f> from the equation
0 will have a negative value fa and a positive value fa at the edge
of the plane, where x = — A, <p = 0.
Hence the charge on the negative side is —cJcfay and that on
the positive side is ckfa.
If we suppose that a is large compared with A,
If we neglect the exponential terms* in fa we shall find that the
charge on the negative surface exceeds that which it would have
if the superficial density had been uniform and equal to that at a
distance from the boundary, by a quantity equal to the charge on a
strip of breadth A— - with the uniform superficial density.
The total capacity of the part of the plane considered is
24:8 CONJUGATE FUNCTIONS. [203.
The total charge is CF, and the attraction towards the infinite
plane is
A
A
a
g-r
A
The equipotential lines and lines of force are given in Fig. XII.
EXAMPLE VIII. — Theory of a Grating of Parallel Wires. Fig. XIII.
203.] In many electrical instruments a wire grating is used to
prevent certain parts of the apparatus from being electrified by
induction. We know that if a conductor be entirely surrounded
by a metallic vessel at the same potential with itself, no electricity
can be induced on the surface of the conductor by any electrified
body outside the vessel. The conductor, however, when completely
surrounded by metal, cannot be seen, and therefore, in certain cases,
an aperture is left which is covered with a grating of fine wire.
Let us investigate the effect of this grating in diminishing the
effect of electrical induction. We shall suppose the grating to
consist of a series of parallel wires in one plane and at equal
intervals, the diameter of the wires being small compared with the
distance between them, while the nearest portions of the electrified
bodies on the one side and of the protected conductor on the other
are at distances from the plane of the screen, which are considerable
compared with the distance between consecutive wires.
204.] The potential at a distance / from the axis of a straight
wire of infinite length charged with a quantity of electricity A. per
unit of length is F = - 2 A log / + <?. ( 1 )
We may express this in terms of polar coordinates referred to an
axis whose distance from the wire is unity, in which case we must
make /2 = 1 -f 2 r cos 0 + r2, (2)
and if we suppose that the axis of reference is also charged with
the linear density A', we find
V — — A. log (1 — 2r cos 0 + r2) — 2 A' log r + C. (3)
If we now make
- /ylv
r = e a , 0 = - - > (4)
205-] INDUCTION THROUGH A GRATING. 249
then, by the theory of conjugate functions,
/ ^JL z-r t^\
= — Alo \l-2e a cos- - + e a / —
.
log \l-2e a cos- - + e a / — 2 A'loge a +C, (5)
where x and y are rectangular coordinates, will be the value of the
potential due to an infinite series of fine wires parallel to z in the
plane of yz, and passing through points in the axis of x for which
# is a multiple of a.
Each of these wires is charged with a linear density A.
The term involving A' indicates an electrification, producing a
constant force -- - — in the direction of y.
a J
The forms of the equipotential surfaces and lines of force when
A'= 0 are given in Fig. XIII. The equipotential surfaces near the
wires are nearly cylinders, so that we may consider the solution
approximately true, even when the wires are cylinders of a dia
meter which is finite but small compared with the distance between
them.
The equipotential surfaces at a distance from the wires become
more and more nearly planes parallel to that of the grating.
If in the equation we make y = 6lt a quantity large compared
with a, we find approximately,
rx = - — i (A + A') j, C nearly. (6)
Cl
If we next make y = — 12 where b.2 is a negative quantity large
compared with a, we find approximately,
F.2 = - i-2 (A - A') + C nearly. (7)
If c is the radius of the wires of the grating, c being small
compared with a, we may find the potential of the grating itself
by supposing that the surface of the wire coincides with the equi
potential surface which cuts the plane of yz at a distance c from the
axis of z. To find the potential of the grating we therefore put
x = c, and y — 0, whence
205.] We have now obtained expressions representing the elec
trical state of a system consisting of a grating of wires whose
diameter is small compared with the distance between them, and
two plane conducting surfaces, one on each side of the grating,
and at distances which are great compared with the distance
between the wires.
250 CONJUGATE FUNCTIONS.
The surface-density o-j on the first plane is got from the equa-
n(6) 4™i=J = _il(X4.V). (9)
That on the second plane <r2 from the equation (7)
db% a
If we now write a , / - , vc^
and eliminate A and X' from the equations (6), (7), (8), (9), (10),
we find
«, + *,+ ?*ll* = Fil + 2^?- r,- F^, (12)
r, i + 2--FJ. (is)
,
a / a ' a
When the wires are infinitely thin, a becomes infinite, and the
terms in which it is the denominator disappear, so that the case
is reduced to that of two parallel planes without a grating in
terposed.
If the grating is in metallic communication with one of the
planes, say the first, V=. V^ , and the right-hand side of the equation
for o-j becomes V^ — Fz . Hence the density ^ induced on the first
plane when the grating is interposed is to that which would have
been induced on it if the grating were removed, the second plane
2 £ A
being maintained at the same potential, as 1 to 1 H — -
We should have found the same value for the effect of the grating
in diminishing the electrical influence of the first surface on the
second, if we had supposed the grating connected with the second
surface. This is evident since #x and d2 enter into the expression
in the same way. It is also a direct result of the theorem of
Art. 88.
The induction of the one electrified plane on the other through
the grating is the same as if the grating were removed, and the
distance between the planes increased from 3X -|- b2 to
If the two planes are kept at potential zero, and the grating
electrified to a given potential, the quantity of electricity on the
grating will be to that which would be induced on a plane of equal
area placed in the same position as
2 dl 62 is to 2 6l &2 + a (61
206.] METHOD OF APPROXIMATION. 251
This investigation is approximate only when b^ and b.2 are large
compared with #, and when a is large compared with c. The
quantity a is a line which may be of any magnitude. It becomes
infinite when c is indefinitely diminished.
If we suppose c •=. \a there will be no apertures between the
wires of the grating, and therefore there will be no induction
through it. We ought therefore to have for this case a = 0. The
formula (11), however, gives in this case
which is evidently erroneous, as the induction can never be altered
in sign by means of the grating. It is easy, however, to proceed
to a higher degree of approximation in the case of a grating of
cylindrical wires. I shall merely indicate the steps of this process.
Method of Approximation.
206.] Since the wires are cylindrical, and since the distribution
of electricity on each is symmetrical with respect to the diameter
parallel to yy the proper expansion of the potential is of the form
F= tf0logr + 2C^cosi0, (14)
where r is the distance from the axis of one of the wires, and 0 the
angle between r and y, and, since the wire is a conductor, when
r is made equal to the radius V must be constant, and therefore
the coefficient of each of the multiple cosines of 6 must vanish.
For the sake of conciseness let us assume new coordinates £, 17, &c.
such that
a^—Z-nx, arj = 27ry, ap = 2itr, 0 /3 = 2 TT £, &c. (15)
and let Fft = log (e^+0 + r-fo+W — 2 cos£). (16)
Then if we make
(»)
by giving proper values to the coefficients A we may express any
potential which is a function of 77 and cos f, and does not become
infinite except when 77 + /3 = 0 and cos £ = 1 .
When (3 = 0 the expansion of F in terms of p and 0 is
F0 = 2logp + -±zp2cos26-T£rzp*cos46 + &c. (18)
For finite values of ft the expansion of F is
1 4- e~P
. (19)
252 CONJUGATE FUNCTIONS. [206.
In the case of the grating with two conducting planes whose
equations are 77 = — /^ and rj = j32, that of the plane of the grating
being rj = 0, there will be two infinite series of images of the
grating. The first series will consist of the grating itself together
with an infinite series of images on both sides, equal and similarly
electrified. The axes of these imaginary cylinders lie in planes
whose equations are of the form
•n = ± 2*(A+/3a), (20)
n being an integer.
The second series will consist of an infinite series of images for
which the coefficients AG, A^ A^ &c. are equal and opposite to the
same quantities in the grating itself, while A19 A^ &c. are equal
and of the same sign. The axes of these images are in planes whose
equations are of the form
rj = 2/32 + 2*»(/31 + /32), (21)
m being an integer.
The potential due to any finite series of such images will depend
on whether the number of images is odd or even. Hence the
potential due to an infinite series is indeterminate, but if we add to
it the function Brj+C, the conditions of the problem will be sufficient
to determine the electrical distribution.
We may first determine V^ and F"2, the potentials of the two
conducting planes, in terms of the coefficients A0, A1} &c., and of
JB and C. We must then determine o-j and o-2, the surface- density
at any point of these planes. The mean values of a-L and cr2 are
given by the equations
4770-!=^-^, 47T<T2 = JQ + J?. (22)
We must then expand the potentials due to the grating itself
and to all the images in terms of p and cosines of multiples of Qy
adding to the result BpcosB+C.
The terms independent of 6 then give V the potential of the
grating, and the coefficient of the cosine of each multiple of 0
equated to zero gives an equation between the indeterminate co
efficients.
In this way as many equations may be found as are sufficient
to eliminate all these coefficients and to leave two equations to
determine o-x and o-2 in terms of T19 F"2, and F".
These equations will be of the form
F2-F = 47rcr1(a + y) + 4wa2(*2 + o-y). (23)
206.] METHOD OF APPROXIMATION. 253
The quantity of electricity induced on one of the planes protected
by the grating, the other plane being at a given difference of
potential, will be the same as if the plates had been at a distance
— — instead of 6-, +6*.
a-f y
The values of a and y are approximately as follows,
a ( , a 5
a=2^r0g2^-3
,6-i±ii / _A-h _„_»!
(24)
CHAPTER XIII.
ELECTROSTATIC INSTRUMENTS.
On Electrostatic Instruments.
THE instruments which we have to consider at present may be
divided into the following classes :
(1) Electrical machines for the production and augmentation of
electrification.
(2) Multipliers, for increasing electrification in a known ratio.
(3) Electrometers, for the measurement of electric potentials and
charges.
(4) Accumulators, for holding large electrical charges.
Electrical Machines.
207.] In the common electrical machine a plate or cylinder of
glass is made to revolve so as to rub against a surface of leather,
on which is spread an amalgam of zinc and mercury. The surface
of the glass becomes electrified positively and that of the rubber
negatively. As the electrified surface of the glass moves away
from the negative electrification of the rubber it acquires a high
positive potential. It then comes opposite to a set of sharp metal
points in connexion with the conductor of the machine. The posi
tive electrification of the glass induces a negative electrification
of the points, which is the more intense the sharper the points
and the nearer they are to the glass.
When the machine works properly there is a discharge through
the air between the glass and the points, the glass loses part of
its positive charge, which is transferred to the points and so to
the insulated prime conductor of the machine, and to any other
body with which it is in electric communication.
The portion of the glass which is advancing towards the rubber
has thus a smaller positive charge than that which is leaving it
at the same time, so that the rubber, and the conductors in com
munication with it, become negatively electrified.
208.] ELECTROPHORUS. 255
The highly positive surface of the glass where it leaves the
rubber is more attracted by the negative charge of the rubber than
the partially discharged surface wnich is advancing towards the
rubber. The electrical forces therefore act as a resistance to the force
employed in turning the machine. The work done in turning the
machine is therefore greater than that spent in overcoming ordinary
friction and other resistances, and the excess is employed in pro
ducing a state of electrification whose energy is equivalent to this
excess.
The work done in overcoming friction is at once converted into
heat in the bodies rubbed together. The electrical energy may
be also converted either into mechanical energy or into heat.
If the machine does not store up mechanical energy, all the
energy will be converted into heat, and the only difference between
the heat due to friction and that due to electrical action is that the
former is generated at the rubbing surfaces while the latter may be
generated in conductors at a distance *.
We have seen that the electrical charge on the surface of the
glass is attracted by the rubber. If this attraction were sufficiently
intense there would be a discharge between the glass and the
rubber, instead of between the glass and the collecting points. To
prevent this, flaps of silk are attached to the rubber. These become
negatively electrified and adhere to the glass, and so diminish the
potential near the rubber.
The potential therefore increases more gradually as the glass
moves away from the rubber, and therefore at any one point there
is less attraction of the charge on the glass towards the rubber, and
consequently less danger of direct discharge to the rubber.
In some electrical machines the moving part is of ebonite instead
of glass, and the rubbers of wool or fur. The rubber is then elec
trified positively and the prime conductor negatively.
The Electrophorus of Volta.
208.] The electrophorus consists of a plate of resin or of ebonite
backed with metal, and a plate of metal of the same size. An
insulating handle can be screwed to the back of either of these
plates. The ebonite plate has a metal pin which connects the metal
* It is probable that in many cases where dynamical energy is converted into heat
by friction, part of the energy may be first transformed into electrical energy and
then converted into heat as the electrical energy is spent in maintaining currents of
short circuit close to the rubbing surfaces. See SirW. Thomson. ' On the Electro-
dynamic Qualities of Metals.' Phil. Trans., 1856, p. 650.
256 ELECTROSTATIC INSTRUMENTS. [209.
plate with the metal back of the ebonite plate when the two plates
are in contact.
The ebonite plate is electrified negatively by rubbing it with
wool or cat's skin. The metal plate is then brought near the
ebonite by means of the insulating handle. No direct discharge
passes between the ebonite and the metal plate, but the potential
of the metal plate is rendered negative by induction, so that when
it comes within a certain distance of the metal pin a spark passes,
and if the metal plate be now carried to a distance it is found
to have a positive charge which may be communicated to a con
ductor. The metal at the back of the ebonite plate is found to
have a negative charge equal and opposite to the charge of the metal
plate.
In using the instrument to charge a condenser or accumulator
one of the plates is laid on a conductor in communication with
the earth, and the other is first laid on it, then removed and applied
to the electrode of the condenser, then laid on the fixed plate and
the process repeated. If the ebonite plate is fixed the condenser will
be charged positively. If the metal plate is fixed the condenser will
be charged negatively.
The work done by the hand in separating the plates is always
greater than the work done by the electrical attraction during the
approach of the plates, so that the operation of charging the con
denser involves the expenditure of work. Part of this work is
accounted for by the energy of the charged condenser, part is spent
in producing the noise and heat of the sparks, and the rest in
overcoming other resistances to the motion.
On Machines producing Electrification by Mechanical Work.
209.] In the ordinary frictional electrical machine the work done
in overcoming friction is far greater than that done in increasing
the electrification. Hence any arrangement by which the elec
trification may be produced entirely by mechanical work against
the electrical forces is of scientific importance if not of practical
value. The first machine of this kind seems to have been Nicholson's
Revolving Doubler, described in the Philosophical Transactions for
1788 as ( an instrument which by the turning of a Winch produces
the two states of Electricity without friction or communication with
the Earth/
210.] It was by means of the revolving doubler that Volta
succeeded in developing from the electrification of the pile an
210.] THE REVOLTING DOUBLER. 257
electrification capable of affecting- his electrometer. Instruments
on the same principle have been invented independently by Mr.
C. F. Varley*, and SirW, Thomson.
These instruments consist essentially of insulated conductors of
various forms, some fixed and others moveable. The moveable
conductors are called Carriers, and the fixed ones may be called
Inductors, Receivers, and Regenerators. The inductors and receivers
are so formed that when the carriers arrive at certain points in
their revolution they are almost completely surrounded by a con
ducting body. As the inductors and receivers cannot completely
surround the carrier and at the same time allow it to move freely
in and out without a complicated arrangement of moveable pieces,
the instrument is not theoretically perfect without a pair of re
generators, which store up the small amount of electricity which
the carriers retain when they emerge from the receivers.
For the present, however, we may suppose the inductors and
receivers to surround the carrier completely when it is within them,
in which case the theory is much simplified.
We shall suppose the machine to consist of two inductors A and
C, and of two receivers B and D, with two carriers F and G.
Suppose the inductor A to be positively electrified so that its
potential is A, and that the carrier F is within it and is at potential
F. Then, if Q is the coefficient of induction (taken positive) between
A and F3 the quantity of electricity on the carrier will be Q (F—A}.
If the carrier, while within the inductor, is put in connexion with
the earth, then F = 0, and the charge on the carrier will be — QA,
a negative quantity. Let the carrier be carried round till it is
within the receiver B, and let it then come in contact with a spring
so as to be in electrical connexion with B. It will then, as was
shewn in Art. 32, become completely discharged, and will com
municate its whole negative charge to the receiver B.
The carrier will next enter the inductor (7, which we shall suppose
charged negatively. While within C it is put in connexion with
the earth and thus acquires a positive charge, which it carries off
and communicates to the receiver D, and so on.
In this way, if the potentials of the inductors remain always
constant, the receivers B and D receive successive charges, which
are the same for every revolution of the carrier, and thus every
revolution produces an equal increment of electricity in the re
ceivers.
* Specification of Patent, Jan. 27, 1860, No. 206. _-^
VOL. I. S
258 ELECTROSTATIC INSTRUMENTS. [2IO.
But by putting the inductor A in communication with the re
ceiver D, and the inductor C with the receiver _Z?, the potentials
of the inductors will be continually increased, and the quantity
of electricity communicated to the receivers in each revolution will
continually increase.
For instance, let the potential of A and 0 be U, and that of £
and C, F, and when the carrier is within A let the charge on A
and C be #, and that on the carrier z, then, since the potential
of the carrier is zero, being in contact with earth, its charge is
z= — QU. The carrier enters B with this charge and communicates
it to B. If the capacity of B and C is B, their potential will be
changed from V to V— ~~ U.
B
If the other carrier has at the same time carried a charge — QF
from C to I), it will change the potential of A and 0 from U to
Q'
U— -£ V3 if Q' is the coefficient of induction between the carrier
and C, and A the capacity of A and D. If, therefore, Un and Fn
be the potentials of the two inductors after n half revolutions, and
Un+1 and Fn+1 after n+1 half revolutions,
77 77 ^ V
un+l — un — ~J "n»
7.+i = r.- 1 ryn.
If we write jt?2 = -£ and 02 = -^ , we find
-o A
Hence
It appears from these equations that the quantity pU+qT con
tinually diminishes, so that whatever be the initial state of elec
trification the receivers are ultimately oppositely electrified, so that
the potentials of A and B are in the ratio of p to — q.
On the other hand, the quantity pU—qV continually increases,
so that, however little pU may exceed or fall short of q Tat first,
the difference will be increased in a geometrical ratio in each
211.] THE RECIPROCAL ELECTROPHORUS. 259
revolution till the electromotive forces become so great that the
insulation of the apparatus is overcome.
Instruments of this kind may be used for various purposes.
For producing* a copious supply of electricity at a high potential,
as is done by means of Mr. Varley's large machine.
For adjusting the charge of a condenser, as in the case of
Thomson's electrometer, the charge of which can be increased or
diminished by a few turns of a veiy small machine of this kind,
which is then called a Replenishes
For multiplying small differences of potential. The inductors
may be charged at first to an exceedingly small potential, as, for
instance, that due to a thermo-electric pair, then, by turning the
machine, the difference of potentials may be continually multiplied
till it becomes capable of measurement by an ordinary electrometer.
By determining by experiment the ratio of increase of this difference
due to each turn of the machine, the original electromotive force
with which the inductors were charged may be deduced from the
number of turns and the final electrification.
In most of these instruments the carriers are made to revolve
about an axis and to come into the proper positions with respect
to the inductors by turning an axle. The connexions are made by
means of springs so placed that the carriers come in contact with
them at the proper instants.
211.] Sir AY. Thomson*, however, has constructed a machine for
multiplying electrical charges in which the carriers are drops of
water falling out of the inside of an inductor into an insulated
receiver. The receiver is thus continually supplied with electricity
of opposite sign to that of the inductor. If the inductor is electrified
positively, the receiver will receive a continually increasing charge
of negative electricity.
The water is made to escape from the receiver by means of a
funnel, the nozzle of which is almost surrounded by the metal of
the receiver. The drops falling from this nozzle are therefore
nearly free from electrification. Another inductor and receiver of
the same construction are arranged so that the inductor of the
one system is in connexion with the receiver of the other. The
rate of increase of charge of the receivers is thus no longer constant,
but increases in a geometrical progression with the time, the
charges of the two receivers being of opposite signs. This increase
goes on till the falling drops are so diverted from their course by
* Proc. E. S., June 20, 1867.
S 3
260
ELECTROSTATIC INSTRUMENTS.
[212.
the electrical action that they fall outside of the receiver or even
strike the inductor.
In this instrument the energy of the electrification is drawn
from that of the falling drops.
212.] Several other electrical machines have been constructed
in which the principle of electric induction is employed. Of these
the most remarkable is that of Holtz, in which the carrier is a glass
plate varnished with gum-lac and the inductors are pieces of
pasteboard. Sparks are prevented from passing between the parts
of the apparatus by means of two glass plates, one on each side
of the revolving carrier plate. This machine is found to be very
effective, and not to be much affected by the state of the atmo
sphere. The principle is the same as in the revolving doubler and
the instruments developed out of the same idea, but as the carrier
is an insulating plate and the inductors are imperfect conductors,
the complete explanation of the action is. more difficult than in
the case where the carriers are good conductors of known form
and are charged and discharged at definite points.
213.] In the electrical machines already described sparks occur
whenever the carrier comes in
contact with a conductor at a
different potential from its
own.
Now we have shewn that
whenever this occurs there is
a loss of energy, and therefore
the whole work employed in
turning the machine is not con
verted into electrification in an
available form, but part is spent
in producing the heat and noise
of electric sparks.
I have therefore thought it desirable to shew how an electrical
machine may be constructed which is not subject to this loss of
efficiency. I do not propose it as a useful form of machine, but
as an example of the method by which the contrivance called in
heat-engines a regenerator may be applied to an electrical machine
to prevent loss of work.
In the figure let A, B, C, A', 1?, Cf represent hollow fixed
conductors, so arranged that the carrier P passes in succession
within each of them. Of these A, A and B, & nearly surround the
Fig. 17.
2 1 3.] MACHINE WITHOUT SPARKS. 261
carrier when it is at the middle point of its passage, but C, C' do not
cover it so much.
We shall suppose A, J3, C to be connected with a Leyden jar
of great capacity at potential F, and A', £', C' to be connected with
another jar at potential — F'.
P is one of the carriers moving in a circle from A to C", &c.,
and touching in its course certain springs, of which a and a are
connected with A and A' respectively, and e, e' are connected with
the earth.
Let us suppose that when the carrier P is in the middle of A
the coefficient of induction between P and A is —A. The capacity
of P in this position is greater than A, since it is not completely
surrounded by the receiver A. Let it be A + a.
Then if the potential of P is Ut and that of A, F, the charge
on P will be (A + a)U-A7.
Now let P be in contact with the spring a when in the middle
of the receiver A, then the potential of P is F, the same as that
of A, and its charge is therefore a V.
If P now leaves the spring a it carries with it the charge a V.
As P leaves A its potential diminishes, and it diminishes still more
when it comes within the influence of C', which is negatively
electrified.
If when P comes within C its coefficient of induction on C is
— C", and its capacity is C' + c', then, if U is the potential of P
the charge on P is
If C'V'-aV,
then at this point U the potential of P will be reduced to zero.
Let P at this point come in contact with the spring e' which is
connected with the earth. Since the potential of P is equal to that
of the spring there will be no spark at contact.
This conductor C', by which the carrier is enabled to be connected
to earth without a spark, answers to the contrivance called a
regenerator in heat-engines. We shall therefore call it a Re
generator.
Now let P move on, still in contact with the earth-spring /, till
it comes into the middle of the inductor .5, the potential of which
is F. If —B is the coefficient of induction between P and B at
this point, then, since U = 0 the charge on P will be — BV.
When P moves away from the earth-spring it carries this charge
with it. As it moves out of the positive inductor B towards the
262 ELECTROSTATIC INSTRUMENTS. [214.
negative receiver A its potential will be increasingly negative. At
the middle of A', if it retained its charge, its potential would be
A' + af
and if B7 \$ greater than a'V its numerical value will be greater
than that of V . Hence there is some point before P reaches the
middle of A' where its potential is — V. At this point let it come
in contact with the negative receiver-spring a' '. There will be no
spark since the two bodies are at the same potential. Let P move
on to the middle of A ' } still in contact with the spring, and therefore
at the same potential with A. During this motion it communicates
a negative charge to A. At the middle of A it leaves the spring
and carries away a charge —a'V towards the positive regenerator
C, where its potential is reduced to zero and it touches the earth-
spring e. It then slides along the earth-spring into the negative
inductor J?', during which motion it acquires a positive charge B' V
which it finally communicates to the positive receiver A, and the
cycle of operations is repeated.
During this cycle the positive receiver has lost a charge a V and
gained a charge Bf V'. Hence the total gain of positive electricity
is B'V'-aV.
Similarly the total gain of negative electricity is BV—a'V.
By making the inductors so as to be as close to the surface of
the carrier as is consistent with insulation, B and B' may be made
large, and by making the receivers so as nearly to surround the
carrier when it is within them, a and a' may be made very small,
and then the charges of both the Leyden jars will be increased in
every revolution.
The conditions to be fulfilled by the regenerators are
C'V = aV, and CF= a'V.
Since a and af are small the regenerators do not require to be
either large or very close to the carriers.
On Electrometers and Electroscopes.
214.] An electrometer is an instrument by means of which
electrical charges or electrical potentials may be measured. In
struments by means of which the existence of electric charges or
of differences of potential may be indicated, but which are not
capable of affording numerical measures, are called Electroscopes.
An electroscope if sufficiently sensible may be used in electrical
measurements, provided we can make the measurement depend on
2 1 5.] COULOMB'S TORSION BALANCE. 263
the absence of electrification. For instance, if we have two charged
bodies A and £ we may use the method described in Chapter I to
determine which body has the greater charge. Let the body A
be carried by an insulating support into the interior of an insulated
closed vessel C. Let C be connected to earth and again insulated.
There will then be no external electrification on C. Now let A
be removed, and B introduced into the interior of C, and the elec
trification of C tested by an electroscope. If the charge of B is
equal to that of A there will be no electrification, but if it is greater
or less there will be electrification of the same kind as that of B, or
the opposite kind.
Methods of this kind, in which the thing to be observed is the
non-existence of some phenomenon, are called null or zero methods.
They require only an instrument capable of detecting the existence
of the phenomenon.
In another class of instruments for the registration of phe
nomena the instrument may be depended upon to give always the
same indication for the same value of the quantity to be registered,
but the readings of the scale of the instrument are not proportional
to the values of the quantity, and the relation between these
readings and the corresponding value is unknown, except that the
one is some continuous function of the other. Several electrometers
depending on the mutual repulsion of parts of the instrument
which are similarly electrified are of this class. The use of such
instruments is to register phenomena, not to measure them. Instead
of the true values of the quantity to be measured, a series of
numbers is obtained, which may be used afterwards to determine
these values when the scale of the instrument has been properly
investigated and tabulated.
In a still higher class of instruments the scale readings are
proportional to the quantity to be measured, so that all that is
required for the complete measurement of the quantity is a know
ledge of the coefficient by which the scale readings must be
multiplied to obtain the true value of the quantity.
Instruments so constructed that they contain within themselves
the means of independently determining the true values of quan
tities are called Absolute Instruments.
Coulomb's Torsion Balance.
215.] A great number of the experiments by which Coulomb
264 ELECTROSTATIC INSTRUMENTS. [215.
established the fundamental laws of electricity were made by mea
suring the force between two small spheres charged with electricity,
one of which was fixed while the other was held in equilibrium by
two forces, the electrical action between the spheres, and the
torsional elasticity of a glass fibre or metal wire. See Art. 38.
The balance of torsion consists of a horizontal arm of gum-lac,
suspended by a fine wire or glass fibre, and carrying at one end a
little sphere of elder pith, smoothly gilt. The suspension wire is
fastened above to the vertical axis of an arm which can be moved
round a horizontal graduated circle, so as to twist the upper end
of the wire about its own axis any number of degrees.
The whole of this apparatus is enclosed in a case. Another little
sphere is so mounted on an insulating stem that it can be charged
and introduced into the case through a hole, and brought so that
its centre coincides with a definite point in the horizontal circle
described by the suspended sphere. The position of the suspended
sphere is ascertained by means of a graduated circle engraved on
the cylindrical glass case of the instrument.
Now suppose both spheres charged, and the suspended sphere
in equilibrium in a known position such that the torsion-arm makes
an angle 0 with the radius through the centre of the fixed sphere.
The distance of the centres is then 2 a sin \ 0, where a is the radius
of the torsion-arm, and if F is the force between the spheres the
moment of this force about the axis of torsion is Fa cos i 0.
Let both spheres be completely discharged, and let the torsion-
arm now be in equilibrium at an angle $ with the radius through
the fixed sphere.
Then the angle through which the electrical force twisted the
torsion-arm must have been 0 — </>, and if M is the moment of
the torsional elasticity of the fibre, we shall have the equation
Fa cos ^0 = M(0-4>).
Hence, if we can ascertain M, we can determine F} the actual
force between the spheres at the distance 2 a sin \ 0.
To find My the moment of torsion, let / be the moment of inertia
of the torsion-arm, and T the time of a double vibration of the arm
under the action of the torsional elasticity, then
M=
In all electrometers it is of the greatest importance to know
what force we are measuring. The force acting on the suspended
2 1 5.] INFLUENCE OF THE CASE. 265
sphere is due partly to the direct action of the fixed sphere, but
partly also to the electrification, if any, of the sides of the case.
If the case is made of glass it is impossible to determine the
electrification of its surface otherwise than by very difficult mea
surements at every point. If, however, either the case is made
of metal, or if a metallic case which almost completely encloses the
apparatus is placed as a screen between the spheres and the glass
case, the electrification of the inside of the metal screen will depend
entirely on that of the spheres, and the electrification of the glass
case will have no influence on the spheres. In this way we may
avoid any indefiniteness due to the action of the case.
To illustrate this by an example in which we can calculate all
the effects, let us suppose that the case is a sphere of radius b,
that the centre of motion of the torsion-arm coincides with the
centre of the sphere and that its radius is a ; that the charges on
the two spheres are E1 and EZ3 and that the angle between their
positions is 6; that the fixed sphere is at a distance a^ from the
centre, and that r is the distance between the two small spheres.
Neglecting for the present the effect of induction on the dis
tribution of electricity on the small spheres, the force between
them will be a repulsion
and the moment of this force round a vertical axis through the
centre will be
EEl aa1 sin 0
-73—
The image of El due to the spherical surface of the case is a point
b2 b
in the same radius at a distance — with a charge — El — , and the
ai ai
moment of the attraction between E and this image about the axis
of suspension is
a — sin Q
x^L
If b, the radius of the spherical case, is large compared with a
266 ELECTEOSTATIC INSTRUMENTS. \_2l6.
and a13 the distances of the spheres from the centre, we may neglect
the second and third terms of the factor in the denominator. The
whole moment tending to turn the torsion-arm may then be written
Electrometers for the Measurement of Potentials.
216.] In all electrometers the moveable part is a body charged
with electricity, and its potential is different from that of certain
of the fixed parts round it. When, as in Coulomb's method, an
insulated body having a certain charge is used, it is the charge
which is the direct object of measurement. We may, however,
connect the balls of Coulomb's electrometer, by means of fine wires,
with different conductors. The charges of the balls will then
depend on the values of the potentials of these conductors and on
the potential of the case of the instrument. The charge on each
ball will be approximately equal to its radius multiplied by the
excess of its potential over that of the case of the instrument,
provided the radii of the balls are small compared with their
distances from each other and from the sides or opening of the
case.
Coulomb's form of apparatus, however, is not well adapted for
measurements of this kind, owing to the smallness of the force
between spheres at the proper distances when the difference of po
tentials is small. A more convenient form is that of the Attracted
Disk Electrometer. The first electrometers on this principle were
constructed by Sir W. Snow Harris*. They have since been
brought to great perfection, both in theory and construction, by
SirW. Thomson f.
When two disks at different potentials are brought face to face
with a small interval between them there will be a nearly uniform
electrification on the opposite faces and very little electrification
on the backs of the disks, provided there are no other conductors
or electrified bodies in the neighbourhood. The charge on the
positive disk will be approximately proportional to its area, and to
'the difference of potentials of the disks, and inversely as the distance
between them. Hence, by making the areas of the disks large
* Phil Trans. 1834.
t See an excellent report on Electrometers by Sir W. Thomson. Report of the
British Association, Dundee, 1867.
PRINCIPLE OF THE GUARD-RING.
267
and the distance between them small, a small difference of potential
may give rise to a measurable force of attraction.
The mathematical theory of the distribution of electricity over
two disks thus arranged is given at Art. 202, but since it is im
possible to make the case of the apparatus so large that we may
suppose the disks insulated in an infinite space, the indications of
the instrument in this form are not easily interpreted numerically.
217.] The addition of the guard-ring to the attracted disk is one
of the chief improvements which Sir W. Thomson has made on the
apparatus.
Instead of suspending the whole of one of the disks and determ
ining the force acting upon it, a central portion of the disk is
separated from the rest to form the attracted disk, and the outer
ring forming the remainder of the disk is fixed. In this way the
force is measured only on that part of the disk where it is most
regular, and the want of uniformity of the electrification near the
COUNTERPOISE
LENS
Fig. 18.
edge is of no importance, as it occurs on the guard-ring and not
on the suspended part of the disk.
Besides this, by connecting the guard-ring with a metal case
surrounding the back of the attracted disk and all its suspending
apparatus, the electrification of the back of the disk is rendered
268 ELECTROSTATIC INSTRUMENTS. [2 I/.
impossible, for it is part of the inner surface of a closed hollow
conductor all at the same potential.
Thomson's Absolute Electrometer therefore consists essentially
of two parallel plates at different potentials, one of which is made
so that a certain area, no part of which is near the edge of the
plate, is moveable under the action of electric force. To fix our
ideas we may suppose the attracted disk and guard-ring uppermost.
The fixed disk is horizontal, and is mounted on an insulating stem
which has a measurable vertical motion given to it by means of
a micrometer screw. The guard-ring is at least as large as the
fixed disk ; its lower surface is truly plane and parallel to the fixed
disk. A delicate balance is erected on the guard-ring to which
is suspended a light moveable disk which almost fills the circular
aperture in the guard-ring without rubbing against its sides. The
lower surface of the suspended disk must be truly plane, and we
must have the means of knowing when its plane coincides with that
of the lower surface of the guard-ring, so as to form a single plane
interrupted only by the narrow interval between the disk and its
guard-ring.
For this purpose the lower disk is screwed up till it is in contact
with the guard-ring, and the suspended disk is allowed to rest
upon the lower disk, so that its lower surface is in the same plane
as that of the guard-ring. Its position with respect to the guard-
ring is then ascertained by means of a system of fiducial marks.
Sir W. Thomson generally uses for this purpose a black hair
attached to the moveable part. This hair moves up or down just
in front of two black dots on a white enamelled ground and is
viewed along with these dots by means of a piano convex lens with
the plane side next the eye. If the hair as seen through the lens
appears straight and bisects the interval between the black dots
it is said to be in its sighted position, and indicates that the sus
pended disk with which it moves is in its proper position as regards
height. The horizontality of the suspended disk may be tested by
comparing the reflexion of part of any object from its upper surface
with that of the remainder of the same object from the upper
surface of the guard-ring.
The balance is then arranged so that when a known weight is
placed on the centre of the suspended disk it is in equilibrium
in its sighted position, the whole apparatus being freed from
electrification by putting every part in metallic communication.
A metal case is placed over the guard-ring so as to enclose the
2 1 8.] THOMSON'S ABSOLUTE ELECTROMETER. 269
balance and suspended disk, sufficient apertures being left to see
the fiducial marks.
The guard-ring-, case, and suspended disk are all in metallic
communication with each other, but are insulated from the other
parts of the apparatus.
Now let it be required to measure the difference of potentials
of two conductors. The conductors are put in communication with
the upper and lower disks respectively by means of wires, the
weight is taken off the suspended disk, and the lower disk is
moved up by means of the micrometer screw till the electrical
attraction brings the suspended disk down to its sighted position.
We then know that the attraction between the disks is equal to
the weight which brought the disk to its sighted position.
If W be the numerical value of the weight, and g the force of
gravity, the force is Wg, and if A is the area of the suspended
disk, D the distance between the disks, and T the difference of the
potentials of the disks,
F2 A
. T-,
If the suspended disk is circular, of radius R, and if the radius of
the aperture of the guard-ring is J?', then
* and r=4
218.] Since there is always some uncertainty in determining the
micrometer reading corresponding to D = 0, and since any error
* Let us denote the radius of the suspended disk by P, and that of the aperture
of the guard-ring by R', then the breadth of the annular interval between the
disk and the ring will be B = R'-R.
If the distance between the suspended disk and the large fixed disk is D, and
the difference of potentials between these disks is F, then, by the investigation in
Art. 201, the quantity of electricity on the suspended disk will be
I 8D 8D D + aJ
where a = B1-^^-, or o = 0. 220635 (R'-E).
If the surface of the guard-ring is not exactly in the plane of the surface of
the suspended disk, let us suppose that the distance between the fixed disk and
the guard-ring is not D but D + z — D', then it appears from the investigation in
Art. 225 that there will be an additional charge of electricity near the edge of
the disk on account of its height z above the general surface of the guard-ring.
The whole charge in this case is therefore
270 ELECTROSTATIC INSTRUMENTS. [218.
in the position of the suspended disk is most important when D
is small, Sir W. Thomson prefers to make all his measurements
depend on differences of the electromotive force F. Thus, if V and
V are two potentials, and D and If the corresponding distances,
F-r' = (D-ff)
A
For instance, in order to measure the electromotive force of a
galvanic battery, two electrometers are used.
By means of a condenser, kept charged if necessary by a re-
plenisher, the lower disk of the principal electrometer is maintained
at a constant potential. This is tested by connecting the lower
disk of the principal electrometer with the lower disk of a secondary
electrometer, the suspended disk of which is connected with the
earth. The distance between the disks of the secondary elec
trometer and the force required to bring the suspended disk to
its sighted position being constant, if we raise the potential of the
condenser till the secondary electrometer is in its sighted position,
we know that the potential of the lower disk of the principal
electrometer exceeds that of the earth by a constant quantity which
we may call V.
If we now connect the positive electrode of the battery to earth,
and connect the suspended disk of the principal electrometer to the
negative electrode, the difference of potentials between the disks
will be V+v, if v is the electromotive force of the battery. Let
D be the reading of the micrometer in this case, and let J/ be the
reading when the suspended disk is connected with earth, then
In this way a small electromotive force v may be measured
by the electrometer with the disks at conveniently measurable
distances. When the distance is too small a small change of
absolute distance makes a great change in the force, since the
and in the expression for the attraction we must substitute for A, the area of the
disk, the corrected quantity
where R = radius of suspended disk,
R' = radius of aperture in the guard-ring,
D = distance between fixed and suspended disks,
D' = distance between fixed disk and guard-ring,
a = 0.220635 (K -E).
When a is small compared with D we may neglect the second term, and when
— D is small we may neglect the last term.
2 1 9.] GAUGE ELECTROMETER. 271
force varies inversely as the square of the distance, so that any
error in the absolute distance introduces a large error in the result
unless the distance is large compared with the limits of error of
the micrometer screw.
The effect of small irregularities of form in the surfaces of the
disks and of the interval between them diminish according to the
inverse cube and higher inverse powers of the distance, and what
ever be the form of a corrugated surface, the eminences of which
just reach a plane surface, the electrical effect at any distance
which is considerable compared to the breadth of the corrugations,
is the same as that of a plane at a certain small distance behind
the plane of the tops of the eminences. See Arts. 197, 198.
By means of the auxiliary electrification, tested by the auxiliary
electrometer, a proper interval between the disks is secured.
The auxiliary electrometer may be of a simpler construction, in
which there is no provision for the determination of the force
of attraction in absolute measure, since all that is wanted is to
secure a constant electrification. Such an electrometer may be
called a gauge electrometer.
This method of using an auxiliary electrification besides the elec
trification to be measured is called the Heterostatic method of
electrometry, in opposition to the Idiostatic method in which the
whole effect is produced by the electrification to be measured.
In several forms of the attracted disk electrometer, the attracted
disk is placed at one end of an arm which is supported by being
attached to a platinum wire passing through its centre of gravity
and kept stretched by means of a spring. The other end of the
arm carries the hair which is brought to a sighted position by
altering the distance between the disks, and so adjusting the force
of the electric attraction to a constant value. In these electro
meters this force is not in general determined in absolute measure,
but is known to be constant, provided the torsional elasticity of
the platinum wire does not change.
The whole apparatus is placed in a Leyden jar, of which the inner
surface is charged and connected with the attracted disk and
guard-ring. The other disk is worked by a micrometer screw and
is connected first with the earth and then with the conductor whose
potential is to be measured. The difference of readings multiplied
by a constant to be determined for each electrometer gives the
potential required.
219.] The electrometers already described are not self-acting,
272 ELECTROSTATIC INSTRUMENTS. [2I9'
but require for each observation an adjustment of a micrometer
screw, or some other movement which must be made by the
observer. They are therefore not fitted to act as self- registering in
struments, which must of themselves move into, the proper position.
This condition is fulfilled by Thomson's Quadrant Electrometer.
The electrical principle on which this instrument is founded may
be thus explained : —
A and B are two fixed conductors which may be at the same
or at different potentials. C is a moveable conductor at a high
potential, which is so placed that part of it is opposite to the
surface of A and part opposite to that of J5, and that the proportions
of these parts are altered as C moves.
For this purpose it is most convenient to make C moveable about
an axis, and make the opposed surfaces of A^ of B, and of C portions
of surfaces of revolution about the same axis.
In this way the distance between the surface of C and the
opposed surfaces of A or of IB remains always the same, and the
motion of C in the positive direction simply increases the area
opposed to B and diminishes the area opposed to A.
If the potentials of A and B are equal there will be no force
urging C from A to B, but if the potential of C differs from that
of B more than from that of A, then C will tend to move so as
to increase the area of its surface opposed to B.
By a suitable arrangement of the apparatus this force may be
made nearly constant for different positions of C within certain
limits, so that if C is suspended by a torsion fibre, its Deflexions
will be nearly proportional to the difference of potentials between
A and B multiplied by the difference of the potential of C from
the mean of those of A and B.
C is maintained at a high potential by means of a condenser
provided with a replenisher and tested by a gauge electrometer,
and A and B are connected with the two conductors the difference
of whose potentials is to be measured. The higher the potential
of C the more sensitive is the instrument. This electrification of
C, being independent of the electrification to be measured, places
this electrometer in the heterostatic class.
We may apply to this electrometer the general theory of systems
of conductors given in Arts. 93, 127.
Let A, B, C denote the potentials of the three conductors re
spectively. Let a, 5, c be their respective capacities, p the coefficient
of induction between B and C, q that between C and A, and r that
2I9.]
QUADRANT ELECTROMETER.
273
between A and B. All these coefficients will in general vary with.
the position of Cy and if C is so arranged that the extremities of A
and B are not near those of C as long as the motion of C is confined
within certain limits, we may ascertain the form of these coefficients.
If 0 represents the deflexion of C from A towards B, then the part
of the surface of A opposed to C will diminish as 0 increases.
Hence if A is kept at potential 1 while B and C"are kept at potential
0, the charge on A will be a = a0 — aO, where a0 and a are
constants, and a is the capacity of A.
If A and B are symmetrical, the capacity of B is I — 50 -f a Q.
The capacity of C is not altered by the motion, for the only
effect of the motion is to bring a different part of C opposite to the
interval between A and B. Hence c = <?0.
The quantity of electricity induced on C when B is raised to
potential unity is p •=. p^ — aQ.
The coefficient of induction between A and C is q = qQ-\-aO.
The coefficient of induction between A and B is not altered by
the motion of C, but remains r = r0.
Hence the electrical energy of the system is
Q = \A*a+%B*b + \C*c + BCp + CAq + ABr,
and if 0 is the moment of the force tending to increase 6,
0 = -~ , A, B, C being supposed constant,
du
.0 da . ~0 db -,n dc
da
or
0 = a(A-B] (C-
In the present form of Thomson's Quadrant Electrometer the
conductors A and B are in the form of
a cylindrical box completely divided
into four quadrants, separately insu
lated, but joined by wires so that two
opposite quadrants are connected with
A and the two others with B.
The conductor C is suspended so as
to be capable of turning about a
vertical axis, and may consist of two
opposite flat quadrantal arcs supported
by their radii at their extremities.
In the position of equilibrium these quadrants should be partly
VOL. I. T
Fig. 19.
274 ELECTROSTATIC INSTRUMENTS. [220.
within A and partly within .5, and the supporting radii should
be near the middle of the quadrants of the hollow base, so that
the divisions of the box and the extremities and supports of C
may be as far from each other as possible.
The conductor C is kept permanently at a high potential by
being connected with the inner coating of the Ley den jar which
forms the case of the instrument. £ and A are connected, the first
with the earth, and the other with the body whose potential is to be
measured.
If the potential of this body is zero, and if the instrument be
in adjustment, there ought to be no force tending to make C move,
but if the potential of A is of the same sign as that of C, then
C will tend to move from A to £ with a nearly uniform force, and
the suspension apparatus will be twisted till an equal force is
called into play and produces equilibrium. For deflexions within
certain limits the deflexions of C will be proportional to the
product (A-B)(C-\(A + B)}.
By increasing the potential of C the sensibility of the instrument
may be increased, and for small values of \ (A 4- B) the force will be
nearly proportional to (A—B] C.
On the Measurement of Electric Potential.
220.] In order to determine large differences of potential in ab
solute measure we may employ the attracted disk electrometer, and
compare the attraction with the effect of a weight. If at the same
time we measure the difference of potential of the same conductors
by means of the quadrant electrometer, we shall ascertain the
absolute value of certain readings of the scale of the quadrant
electrometer, and in this way we may deduce the value of the scale
readings of the quadrant electrometer in terms of the potential
of the suspended part, and the moment of torsion of the suspension
apparatus.
To ascertain the potential of a charged conductor of finite size
we may connect the conductor with one electrode of the electro
meter, while the other is connected to earth or to a body of
constant potential. The electrometer reading will give the potential
of the conductor after the division of its electricity between it
and the part of the electrometer with which it is put in contact.
If K denote the capacity of the conductor, and K' that of this part
221.] MEASUREMENT OF POTENTIAL. 275
of the electrometer, and if V, V denote the potentials of these
bodies before making1 contact, then their common potential after
making contact will be
= _ KF+KT
K+K'
Hence the original potential of the conductor was
If the conductor is not large compared with the electrometer,
K' will be comparable with K, and unless we can ascertain the
values of K and K' the second term of the expression will have
a doubtful value. But if we can make the potential of the electrode
of the electrometer very nearly equal to that of the body before
making contact, then the uncertainty of the values of K and K'
will be of little consequence.
If we know the value of the potential of the body approximately,
we may charge the electrode by means of a ' replenished or other
wise to this approximate potential, and the next experiment will
give a closer approximation. In this way we may measure the
potential of a conductor whose capacity is small compared with
that of the electrometer.
To Measure the Potential at any Point in the Air.
221.] First Method. Place a sphere, whose radius is small com
pared with the distance of electrified conductors, with its centre
at the given point. Connect it by means of a fine wire with the
earth, then insulate it, and carry it to an electrometer and ascertain
the total charge on the sphere.
Then, if V be the potential at the given point, and a the
radius of the sphere, the charge on the sphere will be — Pra=Q^
and if V be the potential of the sphere as measured by an elec
trometer when placed in a room whose walls are connected with
the earth, then Q _ y,
whence V+ V = 0,
or the potential of the air at the point where the centre of the
sphere was placed is equal but of opposite sign to the potential of
the sphere after being connected to earth, then insulated, and
brought into a room.
This method has been employed by M. Delmann of Creuznach in
T 2
276 ELECTROSTATIC INSTRUMENTS.
measuring the potential at a certain height above the earth's
surface.
Second Method. We have supposed the sphere placed at the
given point and first connected to earth, and then insulated, and
carried into a space surrounded with conducting matter at potential
zero.
Now let us suppose a fine insulated wire carried from the elec
trode of the electrometer to the place where the potential is to
be measured. Let the sphere be first discharged completely. This
may be done by putting it into the inside of a vessel of the same
metal which nearly surrounds it and making it touch the vessel.
Now let the sphere thus discharged be carried to the end of the
wire and made to touch it. Since the sphere is not electrified it
will be at the potential of the air at the place. If the electrode
wire is at the same potential it will not be affected by the contact,
but if the electrode is at a different potential it will by contact
with the sphere be made nearer to that of the air than it was
before. By a succession of such operations, the sphere being
alternately discharged and made to touch the electrode, the poten
tial of the electrode of the electrometer will continually approach
that of the air at the given point.
222.] To measure the potential of a conductor without touching
it, we may measure the potential of the air at any point in the
neighbourhood of the conductor, and calculate that of the conductor
from the result. If there be a hollow nearly surrounded by the
conductor, then the potential at any point of the air in this hollow
witt be very nearly that of the conductor.
In this way it has been ascertained by Sir W. Thomson that if
two hollow conductors, one of copper and the other of zinc, are
in metallic contact, then the potential of the air in the hollow
surrounded by zinc is positive with reference to that of the air
in the hollow surrounded by copper.
Third Method. If by any means we can cause a succession of
-small bodies to detach themselves from the end of the electrode,
the potential of the electrode will approximate to that of the sur
rounding air. This may be done by causing shot, filings, sand, or
water to drop out of a funnel or pipe connected with the electrode.
The point at which the potential is measured is that at which
the stream ceases to be continuous and breaks into separate parts
or drops.
Another convenient method is to fasten a slow match to the
223.] THEORY OF THE PROOF PLANE. 277
electrode. The potential is very soon made equal to that of the
air at the burning end of the match. Even a fine metallic point
is sufficient to create a discharge by means of the particles of the
air when the difference of potentials is considerable, but if we
wish to reduce this difference to zero, we must use one of the
methods stated above.
If we only wish to ascertain the sign of the difference of the
potentials at two places, and not its numerical value, we may cause
drops or filings to be discharged at one of the places from a nozzle
connected with the other place, and catch the drops or filings
in an insulated vessel. Each drop as it falls is charged with a
certain amount of electricity, and it is completely discharged into
the vessel. The charge of the vessel therefore is continually ac
cumulating, and after a sufficient number of drops have fallen, the
charge of the vessel may be tested by the roughest methods. The
sign of the charge is positive if the potential of the nozzle is positive
relatively to that of the surrounding air.
MEASUREMENT OF SURFACE-DENSITY OF ELECTRIFICATION.
Theory of the Proof Plane.
223.] In testing the results of the mathematical theory of the
distribution of electricity on the surface of conductors, it is necessary
to be able to measure the surface-density at different points of
the conductor. For this purpose Coulomb employed a small disk
of gilt paper fastened to an insulating stem of gum-lac. He ap
plied this disk to various points of the conductor by placing it
so as to coincide as nearly as possible with the surface of the
conductor. He then removed it by means of the insulating stem,
and measured the charge of the disk by means of his electrometer.
Since the surface of the disk, when applied to the conductor,
nearly coincided with that of the conductor, he concluded that
the surface-density on the outer surface of the disk was nearly
equal to that on the surface of the conductor at that place, and that
the charge on the disk when removed was nearly equal to that
on an area of the surface of the conductor equal to that of one side
of the disk. This disk, when employed in this way, is called
Coulomb's Proof Plane.
As objections have been raised to Coulombs use of the proof
plane, I shall make some remarks on the theory of the experiment.
278 ELECTROSTATIC INSTRUMENTS. [224.
The experiment consists in bringing- a small conducting body
into contact with the surface of the conductor at the point where
the density is to be measured, and then removing the body and
determining its charge.
"We have first to shew that the charge on the small body when
in contact with the conductor is proportional to the surface-
density which existed at the point of contact before the small body
was placed there.
We shall suppose that all the dimensions of the small body, and
especially its dimension in the direction of the normal at the point
of contact, are small compared with either of the radii of curvature
of the conductor at the point of contact. Hence the variation of
the resultant force due to the conductor supposed rigidly electrified
within the space occupied by the small body may be neglected,
and we may treat the surface of the conductor near the small body
as a plane surface.
Now the charge which the small body will take by contact with
a plane surface will be proportional to the resultant force normal
to the surface, that is, to the surface-density. We shall ascertain
the amount of the charge for particular forms of the body.
We have next to shew that when the small body is removed no
spark will pass between it and the conductor, so that it will carry
its charge with it. This is evident, because when the bodies are
in contact their potentials are the same, and therefore the density
on the parts nearest to the point of contact is extremely small.
When the small body is removed to a very short distance from
the conductor, which we shall suppose to be electrified positively,
then the electrification at the point nearest to the small body is
no longer zero but positive, but, since the charge of the small body
is positive, the positive electrification close to the small body will
be less than at other neighbouring points of the surface. Now
the passage of a spark depends in general on the magnitude of the
resultant force, and this on the surface-density. Hence, since we
suppose that the conductor is not so highly electrified as to be
discharging electricity from the other parts of its surface, it will
not discharge a spark to the small body from a part of its surface
which we have shewn to have a smaller surface-density.
224.] We shall now consider various forms of the small body.
Suppose it to be a small hemisphere applied to the conductor so
as to touch it at the centre of its flat side.
Let the conductor be a large sphere, and let us modify the form
225.] THE PROOF PLANE. 279
of the hemisphere so that its surface is a little more than a hemi
sphere, and meets the surface of the sphere at right angles. Then
we have a case of which we have already obtained the exact solution.
See Art. 168.
If A and B be the centres of the two spheres cutting each other
at right angles, DD' a diameter of the circle of intersection, and C
the centre of that circle, then if V is the potential of a conductor
whose outer surface coincides with that of the two spheres, the
quantity of electricity on the exposed surface of the sphere A is
and that on the exposed surface of the sphere B is
4 7 (AD + BD + BC - CD - AC\
the total charge being the sum of these, or
If a and /3 are the radii of the spheres, then, when a is large
compared with £, the charge on B is to that on A in the ratio of
!5o+i£+i $+•*>*»•
Now let & be the uniform surface-density on A when B is re
moved, then the charge on A is
4 TT a2 <r,
and therefore the charge on B is
37r/32o-(l +i^ +&C.),
v 3 a '
or, when B is very small compared with a, the charge on the
hemisphere B is equal to three times that due to a surface-density a-
extending over an area equal to that of the circular base of the
hemisphere.
It appears from Art. 175 that if a small sphere is made to touch
an electrified body, and is then removed to a distance from it, the
mean surface-density on the sphere is to the surface-density of the
body at the point of contact as 7r2 is to 6, or as 1.645 to 1.
225.] The most convenient form for the proof plane is that of
a circular disk. We shall therefore shew how the charge on a
circular disk laid on an electrified surface is to be measured.
For this purpose we shall construct a value of the potential
function so that one of the equipotential surfaces resembles a circular
flattened protuberance whose general form is somewhat like that of
a disk lying on a plane.
280 ELECTROSTATIC INSTRUMENTS. [225-
Let o- be the surface-density of a plane, which we shall suppose
to be that of xy.
The potential due to this electrification will be
y '=—477 0-2.
Now let two disks of radius a be rigidly electrified with surface-
densities — (/ and +</. Let the first of these be placed on the plane
of xy with its centre at the origin, and the second parallel to it at
the very small distance c.
Then it may be shewn, as we shall see in the theory of mag
netism, that the potential of the two disks at any point is a) a-' c,
where &> is the solid angle subtended by the edge of either disk at
the point. Hence the potential of the whole system will be
V = — 4 TT <T Z-\-<&<T c.
The forms of the equipotential surfaces and lines of induction
are given on the left-hand side of Fig. XX, at the end of Vol. II.
Let us trace the form of the surface for which V = 0. This
surface is indicated by the dotted line.
Putting the distance of any point from the axis of z = r, then,
when r is much less than «, and z is small,
£0 = 2 77— 27T- +&C.
a
Hence, for values of r considerably less than a, the equation of
the zero equipotential surface is
£ (*
0 =— 4 7T (TZ+2 77 <r'tf— 27T (/ --- (- &C. j
CT C
or zn =
-
a,
Hence this equipotential surface near the axis is nearly flat.
Outside the disk, where r is greater than a, co is zero when z is
zero, so that the plane of xy is part of the equipotential surface.
To find where these two parts of the surface meet, let us find at
dV
what point of this plane -^- = 0.
When r is very nearly equal to a
dV 2</c
-7- = — 4 TT oH -- •
dz r—a,
Hence, when
dV </c
The equipotential surface V = 0 is therefore composed of a disk-
226.] ' ACCUMULATORS. 281
like figure of radius r0, and nearly uniform thickness z0, and of the
part of the infinite plane of xy which lies beyond this figure.
The surface-integral over the whole disk gives the charge of
electricity on it. It may be found, as in the theory of a circular
current in Part IV, to be
Q = 4 TT a </ c {log 2}-(-7ro-r02.
r0 — a
The charge on an equal area of the plane surface is TT a- r02, hence
the charge on the disk exceeds that on an equal area of the plane
in the ratio of z , Birr .
1 4- 8 — log to unity,
T Z
where z is the thickness and r the radius of the disk, z being sup
posed small compared with r.
On Electric Accumulators and the Measurement of Capacity.
226.] An Accumulator or Condenser is an apparatus consisting
of two conducting surfaces separated by an insulating dielectric
medium.
A Ley den jar is an accumulator in which an inside coating of
tinfoil is separated from the outside coating by the glass of which
the jar is made. The original Leyden phial was a glass vessel
containing- water which was separated by the glass from the hand
which held it.
The outer surface of any insulated conductor may be considered
as one of the surfaces of an accumulator, the other being the earth
or the walls of the room in which it is placed, and the intervening
air being the dielectric medium.
The capacity of an accumulator is measured by the quantity of
electricity with which the inner surface must be charged to make
the difference between the potentials of the surfaces unity.
Since every electrical potential is the sum of a number of parts
found by dividing each electrical element by its distance from a
point, the ratio of a quantity of electricity to a potential must
have the dimensions of a line. Hence electrostatic capacity is a
linear quantity, or we may measure it in feet or metres without
ambiguity.
In electrical researches accumulators are used for two principal
purposes, for receiving and retaining large quantities of electricity
in as small a compass as possible, and for measuring definite quan
tities of electricity by means of the potential to which they raise
the accumulator.
282 ELECTROSTATIC INSTRUMENTS. [227-
For the retention of electrical charges nothing has been devised
more perfect than the Leyden jar. The principal part of the loss
arises from the electricity creeping along the damp uncoated surface
of the glass from the one coating to the other. This may be checked
in a great degree by artificially drying the air within the jar, and
by varnishing the surface of the glass where it is exposed to the
atmosphere. In Sir W. Thomsons electroscopes there is a very
small percentage of loss from day to day, and I believe that none
of this loss can be traced to direct conduction either through air
or through glass when the glass is good, but that it arises chiefly
from superficial conduction along the various insulating stems and
glass surfaces of the instrument.
In fact, the same electrician has communicated a charge to
sulphuric acid in a large bulb with a long neck, and has then her
metically sealed the neck by fusing it, so that the charge was com
pletely surrounded by glass, and after some years the charge was
found still to be retained.
It is only, however, when cold, that glass insulates in this
way, for the charge escapes at once if the glass is heated to
a temperature below 100°C.
When it is desired to obtain great capacity in small compass,
accumulators in which the dielectric is sheet caoutchouc, mica, or
paper impregnated with paraffin are convenient.
227.] For accumulators of the second class, intended for the
measurement of quantities of electricity, all solid dielectrics must be
employed with great caution on account of the property which they
possess called Electric Absorption.
The only safe dielectric for such accumulators is air, which has
this inconvenience, that if any dust or dirt gets into the narrow
space between the opposed surfaces, which ought to be occupied only
by air, it not only alters the thickness of the stratum of air, but
may establish a connexion between the opposed surfaces, in which
case the accumulator will not hold a charge.
To determine in absolute measure, that is to say in feet or metres,
the capacity of an accumulator, we must either first ascertain its
form and size, and then solve the problem of the distribution of
electricity on its opposed surfaces, or we must compare its capacity
with that of another accumulator, for which this problem has been
solved.
As the problem is a very difficult one, it is best to begin with an
accumulator constructed of a form for which the solution is known.
228.] MEASUREMENT OF CAPACITY. 283
Thus the capacity of an insulated sphere in an unlimited space is
known to be measured by the radius of the sphere.
A sphere suspended in a room was actually used by MM. Kohl-
ran sch and Weber, as an absolute standard with which they com
pared the capacity of other accumulators.
The capacity, however, of a sphere of moderate size is so small
when compared with the capacities of the accumulators in common
use that the sphere is not a convenient standard measure.
Its capacity might be greatly increased by surrounding the
sphere with a hollow concentric spherical surface of somewhat
greater radius. The capacity of the inner surface is then a fourth
proportional to the thickness of the stratum of air and the radii of
the two surfaces.
Sir W. Thomson has employed this arrangement as a standard of
capacity, but the difficulties of working the surfaces truly spherical,
of making them truly concentric, and of measuring their distance
and their radii with sufficient accuracy, are considerable.
We are therefore led to prefer for an absolute measure of capacity
a form in which the opposed surfaces are parallel planes.
The accuracy of the surface of the planes can be easily tested,
and their distance can be measured by a micrometer screw, and
may be made capable of continuous variation, which is a most
important property of a measuring instrument.
The only difficulty remaining arises from the fact that the planes
must necessarily be bounded, and that the distribution of electricity
near the boundaries of the planes has not been rigidly calculated.
It is true that if we make them equal circular disks, whose radius
is large compared with the distance between them, we may treat
the edges of the disks as if they were straight lines, and calculate
the distribution of electricity by the method due to Helmholtz, and
described at Art. 202. But it will be noticed that in this case
part of the electricity is distributed on the back of each disk, and
that in the calculation it has been supposed that there are no
conductors in the neighbourhood, which is not and cannot be the
case in a small instrument.
228.] We therefore prefer the following arrangement, due to
Sir W. Thomson, which we may call the Guard-ring arrangement,
by means of which the quantity of electricity on an insulated disk
may be exactly determined in terms of its potential.
284:
ELECTROSTATIC INSTRUMENTS.
[228.
1 M 1 1
U B /
ft <*<y
n *
G
A
G
B
Fig. 20.
The Guard-ring Accumulator.
Bb is a cylindrical vessel of conducting material of which the
outer surface of the upper face is accurately plane. This upper
surface consists of two parts,
a disk A, and a broad ring
BB surrounding the disk,
separated from it by a very
small interval all round, just
sufficient to prevent sparks
passing. The upper surface
of the disk is accurately in
the same plane with that of
the guard-ring. The disk is
supported by pillars of insulating material GG. C is a metal disk,
the under surface of which is accurately plane and parallel to BB.
The disk C is considerably larger than A. Its distance from A
is adjusted and measured by means of a micrometer screw, which
is not given in the figure.
This accumulator is used as a measuring instrument as follows : —
Suppose C to be at potential zero, and the disk A and vessel Bb
both at potential V. Then there will be no electrification on the
back of the disk because the vessel is nearly closed and is all at the
same potential. There will be very little electrification on the
edges of the disk because BB is at the same potential with the
disk. On the face of the disk the electrification will be nearly
uniform, and therefore the whole charge on the disk will be almost
exactly represented by its area multiplied by the surface-density on
a plane, as given at Art. 124.
In fact, we learn from the investigation at Art. 201 that the
charge on the disk is
( 8A SA A+a)
where R is the radius of the disk, R' that of the hole in the guard-
ring, A the distance between A and (7, and a a quantity which
cannot exceed R'-Rl^2-*
If the interval between the disk and the guard-ring is small
compared with the distance between A and C, the second term will
be very small, and the charge on the disk will be nearly
8A
229.] COMPARISON OF CAPACITIES. 285
Now let the vessel Bb be put in connexion with the earth. The
charge on the disk A will no longer be uniformly distributed, but it
will remain the same in quantity, and if we now discharge A we
shall obtain a quantity of electricity, the value of which we know
in terms of V, the original difference of potentials and the measur
able quantities E, R and A.
On the Comparison of the Capacity of Accumulators.
229.] The form of accumulator which is best fitted to have its
capacity determined in absolute measure from the form and dimen
sions of its parts is not generally the most suitable for electrical
experiments. It is desirable that the measures of capacity in actual
use should be accumulators having only two conducting surfaces, one
of which is as nearly as possible surrounded by the other. The
guard-ring accumulator, on the other hand, has three independent
conducting portions which must be charged and discharged in a
certain order. Hence it is desirable to be able to compare the
capacities of two accumulators by an electrical process, so as to test
accumulators which may afterwards serve as secondary standards.
I shall first shew how to test the equality of the capacity of two
guard-ring accumulators.
Let A be the disk, B the guard-ring with the rest of the con
ducting vessel attached to it, and C the large disk of one of these
accumulators, and let A', B ', and C' be the corresponding parts of
the other.
If either of these accumulators is of the more simple kind, having
only two conductors, we have only to suppress B or B', and to
suppose A to be the inner and C the outer conducting surface. C
in this case being understood to surround A.
Let the following connexions be made.
Let B be kept always connected with C", and J? with C, that is,
let each guard-ring be connected with the large disk of the other
condenser.
(1) Let A be connected with B and C' and with /, the electrode
of a Ley den jar, and let A be connected with Bf and C and with
the earth.
(2) Let A, B, and C' be insulated from /.
(3) Let A be insulated from B and C", and A' from Bf and C' '.
(4) Let B and C' be connected with B* and C and with the
earth.
(5) Let A be connected with A'.
286 ELECTROSTATIC INSTRUMENTS. [229.
(6) Let A and A' be connected with an electroscope E.
We may express these connexions as follows : —
(1) Q = C=B' = A' | A=£=C'=J.
(2) 0 = C=&=A' | A=£=C'\J.
(3) 0 = C=& | A' | A\£=C'.
(4) 0 = C=ff\A' | -4 |-B=C"=0.
(5) 0 = (7=^|^ = A\£=C'=0.
(6) 0 = C=ff | ^= J0= ^ | £=<?'= 0.
Here the sign of equality expresses electrical connexion, and the
vertical stroke expresses insulation.
In ( 1 ) the two accumulators are charged oppositely, so that A is
positive and A' negative, the charges on A and A' being uniformly
distributed on the upper surface opposed to the large disk of each
accumulator.
In (2) the jar is removed, and in (3) the charges on A and A' are
insulated.
In (4) the guard-rings are connected with the large disks, so that
the charges on A and A', though unaltered in magnitude, are now
distributed over their whole surface.
In (5) A is connected with A'. If the charges are equal and of
opposite signs, the electrification will be entirely destroyed, and
in (6) this is tested by means of the electroscope JE.
The electroscope E will indicate positive or negative electrification
according as A or A' has the greater capacity.
By means of a key of proper construction, the whole of these
operations can be performed in due succession in a very small
fraction of a second, and the capacities adjusted till no electri
fication can be detected by the electroscope, and in this way the
capacity of an accumulator may be adjusted to be equal to that of
any other, or to the sum of the capacities of several accumulators,
so that a system of accumulators may be formed, each of which has
its capacity determined in absolute measure, i. e. in feet or in metres,
while at the same time it is of the construction most suitable for
electrical experiments.
This method of comparison will probably be found useful in
determining the specific capacity for electrostatic induction of
different dielectrics in the form of plates or disks. If a disk of
the dielectric is interposed between A and C, the disk being con
siderably larger than A, then the capacity of the accumulator will
229.] SPECIFIC INDUCTIVE CAPACITY. 287
be altered and made equal to that of the same accumulator when A
and C are nearer together. If the accumulator with the dielectric
plate, and with A and C at distance x, is of the same capacity as
the same accumulator without the dielectric, and with A and C at
distance x ', then, if a is the thickness of the plate, and K its specific
dielectric inductive capacity referred to air as a standard,
—x
The combination of three cylinders, described in Art. 127, has
been employed by Sir TV. Thomson as an accumulator whose capa
city may be increased or diminished by measurable quantities.
The experiments of MM. Gibson and Barclay with this ap
paratus are described in the Proceedings of the Royal Society, Feb. 2,
1871, and Phil. Trans., 1871, p. 573. They found the specific in
ductive capacity of paraffin to be 1.975, that of air being unity,
PART II.
ELECTED KINEMATICS.
CHAPTEE I,
THE ELECTRIC CURRENT.
230.] WE have seen, in Art. 45, that when a conductor is in
electrical equilibrium the potential at every point of the conductor
must be the same.
If two conductors A and B are charged with electricity so that
the potential of A is higher than that of B, then, if they are put
in communication by means of a metallic wire C touching both of
them, part of the charge of A will be transferred to B, and the
potentials of A and B will become in a very short time equalized.
231.] During this process certain phenomena are observed in
the wire C, which are called the phenomena of the electric conflict
or current.
The first of these phenomena is the transference of positive
electrification from A to B and of negative electrification from B
to A. This transference may be also effected in a slower manner
by bringing a small insulated body into contact with A and B
alternately. By this process, which we may call electrical con
vection, successive small portions of the electrification of each body
are transferred to the other. In either case a certain quantity of
electricity, or of the state of electrification, passes from one place
to another along a certain path in the space between the bodies.
Whatever therefore may be our opinion of the nature of elec
tricity, we must admit that the process which we have described
constitutes a current of electricity. This current may be described
232.] THE VOLTAIC BATTERY. 289
as a current of positive electricity from A to .5, or a current of
negative electricity from B to A, or as a combination of these two
currents.
According to Fechner's and Weber's theory it is a combination
of a current of positive electricity with an exactly equal current
of negative electricity in the opposite direction through the same
substance. It is necessary to remember this exceedingly artificial
hypothesis regarding the constitution of the current in order to
understand the statement of some of Weber's most valuable ex
perimental results.
If, as in Art. 36, we suppose P units of positive electricity
transferred from A to B , and N units of negative electricity trans
ferred from B to A in unit of time, then, according to Weber's
theory, P=N, and P or N is to be taken as the numerical measure
of the current.
We, on the contrary, make no assumption as to the relation
between P and N, but attend only to the result of the current,
namely, the transference of P + Ar of positive electrification from A
to j5, and we shall consider P-f N the true measure of the current.
The current, therefore, which Weber would call 1 we shall call 2.
On Steady Currents.
232.] In the case of the current between two insulated con
ductors at different potentials the operation is soon brought to
an end by the equalization of the potentials of the two bodies,
and the current is therefore essentially a Transient current.
But there are methods by which the difference of potentials of
the conductors may be maintained constant, in which case the
current will continue to flow with uniform strength as a Steady
Current.
The Voltaic Battery.
The most convenient method of producing a steady current is by
means of the Voltaic Battery.
For the sake of distinctness we shall describe Daniell's Constant
Battery : —
A solution of sulphate of zinc is placed in a cell of porous earth
enware, and this cell is placed in a vessel containing a saturated
solution of sulphate of copper. A piece of zinc is dipped into the
sulphate of zinc, and a piece of copper is dipped into the sulphate
of copper. Wires are soldered to the zinc and to the copper above
VOL. i. u
290 THE ELECTRIC CURRENT. [233*
the surface of the liquid. This combination is called a cell or
element of Daniell's battery. See Art. 272.
233.] If the cell is insulated by being placed on a non-con
ducting stand, and if the wire connected with the copper is put
in contact with an insulated conductor A, and the wire connected
with the zinc is put in contact with J3, another insulated conductor
of the same metal as A, then it may be shewn by means of a delicate
electrometer that the potential of A exceeds that of B by a certain
quantity. This difference of potentials is called the Electromotive
Force of the Daniell's Cell.
If A and B are now disconnected from the cell and put in
communication by means of a wire, a transient current passes
through the wire from A to J3, and the potentials of A and B
become equal. A and B may then be charged again by the cell,
and the process repeated as long as the cell will work. But if
A and B be connected by means of the wire C, and at the same
time connected with the battery as before, then the cell will main
tain a constant current through C, and also a constant difference
of potentials between A and B. This difference will not, as we
shall see, be equal to the whole electromotive force of the cell, for
part of this force is spent in maintaining the current through the
cell itself.
A number of cells placed in series so that the zinc of the first
cell is connected by metal with the copper of the second, and
so on, is called a Voltaic Battery. The electromotive force of
such a battery is the sum of the electromotive forces of the cells
of which it is composed. If the battery is insulated it may be
charged with electricity as a whole, but the potential of the copper
end will always exceed that of the zinc end by the electromotive
force of the battery, whatever the absolute value of either of these
potentials may be. The cells of the battery may be of very various
construction, containing different chemical substances and different
metals, provided they are such that chemical action does not go
on when no current passes.
234.] Let us now consider a voltaic battery with its ends insulated
from each other. The copper end will be positively or vitreously
electrified, and the zinc end will be negatively or resinously electrified.
Let the two ends of the battery be now connected by means
of a wire. An electric current will commence, and will in a very
short time attain a constant value. It is then said to be a Steady
Current.
236.] ELECTKOLYSIS. 291
Properties of the Current.
235.] The current forms a closed circuit in the direction from
copper to zinc through the wires, and from zinc to copper through
the solutions.
If the circuit be broken by cutting any of the wires which
connect the copper of one cell with the zinc of the next in order, the
current will be stopped, and the potential of the end of the wire
in connexion with the copper will be found to exceed that of the
end of the wire in connexion with the zinc by a constant quantity,
namely, the total electromotive force of the circuit.
Electrolytic Action of the Current.
236.] As long as the circuit is broken no chemical action goes
on in the cells, but as soon as the circuit is completed, zinc is
dissolved from the zinc in each of the Daniell's cells, and copper is
deposited on the copper.
The quantity of sulphate of zinc increases, and the quantity of
sulphate of copper diminishes unless more is constantly supplied.
The quantity of zinc dissolved and also that of copper deposited is
the same in each of the Daniell's cells throughout the circuit, what
ever the size of the plates of the cell, and if any of the cells be of a
different construction, the amount of chemical action in it bears
a constant proportion to the action in the Daniell's cell. For
instance, if one of the cells consists of two platinum plates dipped
into sulphuric acid diluted with water, oxygen will be given off
at the surface of the plate where the current enters the liquid,
namely, the plate in metallic connexion with the copper of Daniell's
cell, and hydrogen at the surface of the plate where the current
leaves the liquid, namely, the plate connected with the zinc of
Daniell's cell.
The volume of the hydrogen is exactly twice the volume of the
oxygen given off in the same time, and the weight of the oxygen is
exactly eight times the weight of the hydrogen.
In every cell of the circuit the weight of each substance dissolved,
deposited, or decomposed is equal to a certain quantity called the
electrochemical equivalent of that substance, multiplied by the
strength of the current and by the time during which it has
been flowing.
For the experiments which established this principle, see the
seventh and eighth series of Faraday's Experimental Researches;
T: 2
292 THE ELECTRIC CURRENT. [237.
and for an investigation of the apparent exceptions to the rule, see
Miller's Chemical Physics and Wiedemann's Galvanismus.
237.] Substances which are decomposed in this way are called
Electrolytes. The process is called Electrolysis. The places where
the current enters and leaves the electrolyte are called Electrodes.
Of these the electrode by which the current enters is called the
Anode, and that by which it leaves the electrolyte is called the
Cathode. The components into which the electrolyte is resolved
are called Ions : that which appears at the anode is called the
Anion, and that which appears at the cathode is called the Cation.
Of these terms, which were, I believe, invented by Faraday with
the help of Dr. Whewell, the first three, namely, electrode, elec
trolysis, and electrolyte have been generally adopted, and the mode
of conduction of the current in which this kind of decomposition
and transfer of the components takes place is called Electrolytic
Conduction.
If a homogeneous electrolyte is placed in a tube of variable
section, and if the electrodes are placed at the ends of this tube,
it is found that when the current passes, the anion appears at
the anode and the cation at the cathode, the quantities of these
ions being electrochemically equivalent, and such as to be together
equivalent to a certain quantity of the electrolyte. In the other
parts of the tube, whether the section be large or small, uniform
or varying, the composition of the electrolyte remains unaltered.
Hence the amount of electrolysis which takes place across every
section of the tube is the same. Where the section is small the
action must therefore be more intense than where the section is
large, but the total amount of each ion which crosses any complete
section of the electrolyte in a given time is the same for all sections.
The strength of the current may therefore be measured by the
amount of electrolysis in a given time. An instrument by which
the quantity of the electrolytic products can be readily measured
is called a Voltameter.
The strength of the current, as thus measured, is the same
at every part of the circuit, and the total quantity of the elec
trolytic products in the voltameter after any given time is pro
portional to the amount of electricity which passes any section in
the same time.
238.] If we introduce a voltameter at one part of the circuit
of a voltaic battery, and break the circuit at another part, we may
suppose the measurement of the current to be conducted thus.
239-] MAGNETIC ACTION. 293
Let the ends of the broken circuit be A and H, and let A be the
anode and B the cathode. Let an insulated ball be made to touch
A and B alternately, it will carry from A to B a certain measurable
quantity of electricity at each journey. This quantity may be
measured by an electrometer, or it may be calculated by mul
tiplying the electromotive force of the circuit by the electrostatic
capacity of the ball. Electricity is thus carried from A to B on the
insulated ball by a process which may be called Convection. At
the same time electrolysis goes on in the voltameter and in the
c^lls of the battery, and the amount of electrolysis in each cell may
be compared with the amount of electricity carried across by the
insulated ball. The quantity of a substance which is electrolysed
by one unit of electricity is called an Electrochemical equivalent
of that substance.
This experiment would be an extremely tedious and troublesome
one if conducted in this way with a ball of ordinary magnitude
and a manageable battery, for an enormous number of journeys
would have to be made before an appreciable quantity of the electro
lyte was decomposed. The experiment must therefore be considered
as a mere illustration, the actual measurements of electrochemical
equivalents being conducted in a different way. But the experi
ment may be considered as an illustration of the process of elec
trolysis itself, for if we regard electrolytic conduction as a species
of convection in which an electrochemical equivalent of the anion
travels with negative electricity in the direction of the anode, while
an equivalent of the cation travels with positive electricity in
the direction of the cathode, the whole amount of transfer of elec
tricity being one unit, we shall have an idea of the process of
electrolysis, which, so far as I know, is not inconsistent with known
facts, though, on account of our ignorance of the nature of electricity
and of chemical compounds, it may be a very imperfect repre
sentation of what really takes place.
Magnetic Action of the Current.
239.] Oersted discovered that a magnet placed near a straight
electric current tends to place itself at right angles to the plane
passing through the magnet and the current. See Art. 475.
If a man were to place his body in the line of the current so
that the current from copper through the wire to zinc should flow
from his head to his feet, and if he were to direct his face towards
the centre of the magnet, then that end of the magnet which tends
294 THE ELECTEIC CUR11ENT. [240.
to point to the north would, when the current flows, tend to point
towards the man's right hand.
The nature and laws of this electromagnetic action will be dis
cussed when we come to the fourth part of this treatise. What
we are concerned with at present is the fact that the electric
current has a magnetic action which is exerted outside the current,
and by which its existence can be ascertained and its intensity
measured without breaking the circuit or introducing anything into
the current itself.
The amount of the magnetic action has been ascertained to be
strictly proportional to the strength of the current as measured
by the products of electrolysis in the voltameter, and to be quite
independent of the nature of the conductor in which the current
is flowing, whether it be a metal or an electrolyte.
240.] An instrument which indicates the strength of an electric
current by its magnetic effects is called a Galvanometer.
Galvanometers in general consist of one or more coils of silk-
covered wire within which a magnet is suspended with its axis
horizontal. When a current is passed through the wire the magnet
tends to set itself with its axis perpendicular to the plane of the
coils. If we suppose the plane of the coils to be placed parallel
to the plane of the earth's equator, and the current to flow round
the coil from east to west in the direction of the apparent motion
of the sun, then the magnet within will tend to set itself with
its magnetization in the same direction as that of the earth con
sidered as a great magnet, the north pole of the earth being similar
to that end of the compass needle which points south.
The galvanometer is the most convenient instrument for mea
suring the strength of electric currents. We shall therefore assume
the possibility of constructing such an instrument in studying the
laws of these currents, reserving the discussion of the principles of
the instrument for our fourth part. When therefore we say that
an electric current is of a certain strength we suppose that the
measurement is effected by the galvanometer.
CHAPTER II.
CONDUCTION AND KESISTANCE.
241.] IF by means of an electrometer we determine the electric
potential at different points of a circuit in which a constant electric
current is maintained, we shall find that in any portion of the
circuit consisting of a single metal of uniform temperature through
out, the potential at any point exceeds that at any other point
farther on in the direction of the current by a quantity depending
on the strength of the current and on the nature and dimensions
of the intervening portion of the circuit. The difference of the
potentials at the extremities of this portion of the circuit is called
the External electromotive force acting on it. If the portion of
the circuit under consideration is not homogeneous, but contains
transitions from one substance to another, from metals to elec
trolytes, or from hotter to colder parts, there may be, besides the
external electromotive force, Internal electromotive forces which
must be taken into account.
The relations between Electromotive Force, Current, and Resist
ance were first investigated by Dr. G. S. Ohm, in a work published
in 1827, entitled Die Galvanische Kette Mathematisch Bearbeitet,
translated in Taylor's Scientific Memoirs. The result of these in
vestigations in the case of homogeneous conductors is commonly
called ' Ohm's Law.'
Ohm's Laio.
The electromotive force acting between the extremities of any part
of a circuit is the product of the strength of the current and the
Resistance of that part of the circuit.
Here a new term is introduced, the Resistance of a conductor,
which is defined to be the ratio of the electromotive force to
the strength of the current which it produces. The introduction
296 CONDUCTION AND RESISTANCE. [242.
of this term would have been of no scientific value unless Ohm
had shewn,, as he did experimentally, that it corresponds to a real
physical quantity, that is, that it has a definite value which is
altered only when the nature of the conductor is altered.
In the first place, then, the resistance of a conductor is inde
pendent of the strength of the current flowing through it.
In the second place the resistance is independent of the electric
potential at which the conductor is maintained, and of the density
of the distribution of electricity on the surface of the conductor.
It depends entirely on the nature of the material of which the
conductor is composed, the state of aggregation of its parts, and its
temperature.
The resistance of a conductor may be measured to within one
ten thousandth or even one hundred thousandth part of its value,
and so many conductors have been tested that our assurance of the
truth of Ohm's Law is now very high, In the sixth chapter we
shall trace its applications and consequences.
Generation of Heat by the Current.
242.] We have seen that when an electromotive force causes
a current to flow through a conductor, electricity is transferred
from a place of higher to a place of lower potential. If the transfer
had been made by convection, that is, by carrying successive
charges on a ball from the one place to the other, work would have
been done by the electrical forces on the ball, and this might have
been turned to account. It is actually turned to account in a
partial manner in those dry pile circuits where the electrodes have
the form of bells, and the carrier ball is made to swing like a
pendulum between the two bells and strike them alternately. In
this way the electrical action is made to keep up the swinging
of the pendulum and to propagate the sound of the bells to a
distance. In the case of the conducting wire we have the same
transfer of electricity from a place of high to a place of low potential
without any external work being done. The principle of the Con
servation of Energy therefore leads us to look for internal work in
the conductor. In an electrolyte this internal work consists partly
of the separation of its components. In other conductors it is
entirely converted into heat.
The energy converted into heat is in this case the product of
the electromotive force into the quantity of electricity which passes.
But the electromotive force is the product of the current into the
244-] COMPARISON WITH PHENOMENA OF HEAT. 297
resistance, and the quantity of electricity is the product of the
current into the time. Hence the quantity of heat multiplied by
the mechanical equivalent of unit of heat is equal to the square of
the strength of the current multiplied into the resistance and into
the time.
The heat developed by electric currents in overcoming1 the re
sistance of conductors has been determined by Dr. Joule, who
first established that the heat produced in a given time is pro
portional to the square of the current, and afterwards by careful
absolute measurements of all the quantities concerned, verified the
Cation JH=C*Rt,
where / is Joule's dynamical equivalent of heat, H the number of
units of heat, C the strength of the current, R the resistance of the
conductor, and t the time during which the current flows. These
relations between electromotive force, work, and heat, were first fully
explained by Sir W. Thomson in a paper on the application of the
principle of mechanical effect to the measurement of electromotive
forces *.
243.] The analogy between the theory of the conduction of
electricity and that of the conduction of heat is at first sight almost
complete. If we take two systems geometrically similar, and such
that the conductivity for heat at any part of the first is proportional
to the conductivity for electricity at the corresponding part of the
second, and if we also make the temperature at any part of the
first proportional to the electric potential at the corresponding point
of the second, then the flow of heat across any area of the first
will be proportional to the flow of electricity across the corre
sponding area of the second.
Thus, in the illustration we have given, in which flow of elec
tricity corresponds to flow of heat, and electric potential to tem
perature, electricity tends to flow from places of high to places
of low potential, exactly as heat tends to flow from places of high
to places of low temperature.
244.] The theory of potential and that of temperature may
therefore be made to illustrate one another ; there is, however, one
remarkable difference between the phenomena of electricity and
those of heat.
Suspend a conducting body within a closed conducting vessel by
a silk thread, and charge the vessel with electricity. The potential
* PhU. Mag., Dec. 1851.
298 CONDUCTION AND RESISTANCE. [245-
of the vessel and of all within it will be instantly raised, but
however long and however powerfully the vessel be electrified, and
whether the body within be allowed to come in contact with the
vessel or not, no signs of electrification will appear within the
vessel, nor will the body within shew any electrical effect when
taken out.
But if the vessel is raised to a high temperature, the body
within will rise to the same temperature, but only after a con
siderable time, and if it is then taken out it will be found hot,
and will remain so till it has continued to emit heat for some time.
The difference between the phenomena consists in the fact that
bodies are capable of absorbing and emitting heat, whereas they
have no corresponding property with respect to electricity. A body
cannot be made hot without a certain amount of heat being
supplied to it, depending on the mass and specific heat of the body,
but the electric potential of a body may be raised to any extent
in the way already described without communicatiDg any electricity
to the body.
245.] Again, suppose a body first heated and then placed inside
the closed vessel. The outside of the vessel will be at first at the
temperature of surrounding bodies, but it will soon get hot, and
will remain hot till the heat of the interior body has escaped.
It is impossible to perform a corresponding electrical experiment.
It is impossible so to electrify a body, and so to place it in a
hollow vessel, that the outside of the vessel shall at first shew no
signs of electrification but shall afterwards become electrified. It
was for some phenomenon of this kind that Faraday sought in
vain under the name of an absolute charge of electricity.
Heat may be hidden in the interior of a body so as to have no
external action, but it is impossible to isolate a quantity of elec
tricity so as to prevent it from being constantly in inductive
relation with an equal quantity of electricity of the opposite kind.
There is nothing therefore among electric phenomena which
corresponds to the capacity of a body for heat. This follows at
once from the doctrine which is asserted in this treatise, that
electricity obeys the same condition of continuity as an incom
pressible fluid. It is therefore impossible to give a bodily charge
of electricity to any substance by forcing an additional quantity of
electricity into it. See Arts. 61, 111, 329, 334.
CHAPTER III.
ELECTROMOTIVE FORCE BETWEEN BODIES IN CONTACT.
The Potentials of Different Substances in Contact.
246.] IF we define the potential of a hollow conducting vessel
as the potential of the air inside the vessel, we may ascertain this
potential by means of an electrometer as described in Part I,
Art, 222.
If we now take two hollow vessels of different metals, say copper
and zinc, and put them in metallic contact with each other, and
then test the potential of the air inside each vessel, the potential
of the air inside the zinc vessel will be positive as compared with
that inside the copper vessel. The difference of potentials depends
on the nature of the surface of the insides of the vessels, being
greatest when the zinc is bright and when the copper is coated
with oxide.
It appears from this that when two different metals are in
contact there is in general an electromotive force acting from the
one to the other, so as to make the potential of the one exceed
that of the other by a certain quantity. This is Volta's theory of
Contact Electricity.
If we take a certain metal, say copper, as the standard, then
if the potential of iron in contact with copper at the zero potential
is /, and that of zinc in contact with copper at zero is Z, then
the potential of zinc in contact with iron at zero will be Z—I.
It appears from this result, which is true of any three metals,
that the differences of potential of any two metals at the same
temperature in contact is equal to the difference of their potentials
when in contact with a third metal, so that if a circuit be formed
of any number of metals at the same temperature there will be
electrical equilibrium as soon as they have acquired their proper
potentials, and there will be no current kept up in the circuit.
300 CONTACT FOKCE. [247-
247.] If, however, the circuit consist of two metals and an elec
trolyte, the electrolyte, according 'to Volta's theory, tends to reduce
the potentials of the metals in contact with it to equality, so that
the electromotive force at the metallic junction is no longer balanced,
and a continuous current is kept up. The energy of this current
is supplied by the chemical action which takes place between the
electrolyte and the metals.
248.] The electric effect may. however, be produced without
chemical action if by any other means we can produce an equali
zation of the potentials of two metals in contact. Thus, in an
experiment due to Sir W. Thomson*, a copper funnel is placed in
contact with a vertical zinc cylinder, so that when copper filings
are allowed to pass through the funnel, they separate from each
other and from the funnel near the middle of the zinc cylinder,
and then fall into an insulated receiver placed below. The receiver
is then found to be charged negatively, and the charge increases
as the filings continue to pour into it. At the same time the zinc
cylinder with the copper funnel in it becomes charged more and
more positively.
If now the zinc cylinder were connected with the receiver by a
wire, there would be a positive current in the wire from the cylinder
to the receiver. The stream of copper filings, each filing charged
negatively by induction, constitutes a negative current from the
funnel to the receiver, or, in other words, a positive current from
the receiver to the copper funnel. The positive current, therefore,
passes through the air (by the filings) from zinc to copper, and
through the metallic junction from copper to zinc, just as in the
ordinary voltaic arrangement, but in this case the force which keeps
up the current is not chemical action but gravity, which causes the
filings to fall, in spite of the electrical attraction between the
positively charged funnel and the negatively charged filings.
249.] A remarkable confirmation of the theory of contact elec
tricity is supplied by the discovery of Peltier, that, when a current
of electricity crosses the junction of two metals, the junction is
heated when the current is in one direction, and cooled when it
is in the other direction. It must be remembered that a current
in its passage through a metal always produces heat, because it
meets with resistance, so that the cooling effect on the whole
conductor must always be less than the heating effect. We must
therefore distinguish between the generation of heat in each metal,
North British Revieiv, 1864, p. 353 ; and Proc. E. S., June 20, 1867.
249-] PELTIER'S PHENOMENON. 301
due to ordinary resistance, and the generation or absorption of heat
at the junction of two metals. We shall call the first the frictional
generation of heat by the current, and, as we have seen, it is
proportional to the square of the current, and is the same whether
the current be in the positive or the negative direction. The second
we may call the Peltier effect, which changes its sign with that
of the current.
The total heat generated in a portion of a compound conductor
consisting of two metals may be expressed by
H = c*t-
where H is the quantity of heat, / the mechanical equivalent of
unit of heat, R the resistance of the conductor, C the current, and
t the time ; IT being the coefficient of the Peltier effect, that is,
the heat absorbed at the junction due to the passage of unit of
current for unit of time.
Now the heat generated is mechanically equivalent to the work
done against electrical forces in the conductor, that is, it is equal
to the product of the current into the electromotive force producing
it. Hence, if E is the external electromotive force which causes
the current to flow through the conductor,
JH= CUt = RC* t-JU Ct,
whence E = RC-JU.
It appears from this equation that the external electromotive
force required to drive the current through the compound conductor
is less than that due to its resistance alone by the electromotive
force /fl. Hence /n represents the electromotive contact force
at the junction acting in the positive direction.
This application, due to Sir W. Thomson *, of the dynamical
theory of heat to the determination of a local electromotive force
is of great scientific importance, since the ordinary method of
connecting two points of the compound conductor with the elec
trodes of a galvanometer or electroscope by wires would be useless,
owing to the contact forces at the junctions of the wires with
the materials of the compound conductor. In the thermal method,
on the other hand, we know that the only source of energy is the
current of electricity, and that no work is done by the current
in a certain portion of the circuit except in heating that portion
of the conductor. If, therefore, we can measure the amount of the
* Proc. R. S. Edin., Dec. 15, 1851 ; and Trans. R. 8. Edin., 1854.
302 CONTACT FORCE. [250.
current and the amount of heat produced or absorbed, we can
determine the electromotive force required to urge the current
through that portion of the conductor, and this measurement is
entirely independent of the effect of contact forces in other parts of
the circuit.
The electromotive force at the junction of two metals, as de
termined by this method, does not account for Volta's electromotive
force as described in Art. 246. The latter is in general far greater
than that of this Article, and is sometimes of opposite sign. Hence
the assumption that the potential of a metal is to be measured by
that of the air in contact with it must be erroneous, and the greater
part of Volta's electromotive force must be sought for, not at the
junction of the two metals, but at one or both of the surfaces which
separate the metals from the air or other medium which forms the
third element of the circuit.
250.] The discovery by Seebeck of thermoelectric currents in
circuits of different metals with their junctions at different tem
peratures, shews that these contact forces do not always balance
each other in a complete circuit. It is manifest, however, that
in a complete circuit of different metals at uniform temperature the
contact forces must balance each other. For if this were not the
case there would be a current formed in the circuit, and this current
might be employed to work a machine or to generate heat in the
circuit, that is, to do work, while at the same time there is no
expenditure of energy, as the circuit is all at the same temperature,
and no chemical or other change takes place. Hence, if the Peltier
effect at the junction of two metals a and d be represented by Ha&
when the current flows from a to b, then for a circuit of two metals
at the same temperature we must have
and for a circuit of three metals a, 6, c, we must have
nbc+nca+na& = o.
It follows from this equation that the three Peltier effects are not
independent, but that one of them can be deduced from the other
two. For instance, if we suppose c to be a standard metal, and
if we write Pa = /nac and Pb — JUbc, then
JUab = Pa-Pb.
The quantity Pa is a function of the temperature, and depends on
the nature of the metal a.
251.] It has also been shewn by Magnus that if a circuit is
251.] THERMOELECTRIC PHENOMENA. 303
formed of a single metal no current will be formed in it, however
the section of the conductor and the temperature may vary in
different parts.
Since in this case there is conduction of heat and consequent
dissipation of energy, we cannot, as in the former case, consider this
result as self-evident. The electromotive force, for instance, between
two portions of a circuit might have depended on whether the
current was passing from a thick portion of the conductor to a thin
one, or the reverse, as well as on its passing rapidly or slowly from a
hot portion to a cold one, or the reverse, and this would have made
a current possible in an unequally heated circuit of one metal.
Hence, by the same reasoning as in the case of Peltier's phe
nomenon, we find that if the passage of a current through a
conductor of one metal produces any thermal effect which is re
versed when the current is reversed, this can only take place when
the current flows from places of high to places of low temperature,
or the reverse, and if the heat generated in a conductor of one
metal in flowing from a place where the temperature is a? to a
place where it is ?/, is H, then
JH= RCH-SxyCt,
and the electromotive force tending to maintain the current will
be Sxv.
If x, y, z be the temperatures at three points of a homogeneous
circuit, we must have
Svz + Szx + Sxy = 0,
according to the result of Magnus. Hence, if we suppose z to be
the zero temperature, and if we put
QX = SX, and Qy = Syz,
we find Sxy=Qx-Qv,
where Qx is a function of the temperature x, the form of the
function depending on the nature of the metal.
If we now consider a circuit of two metals a and b in which
the temperature is x where the current passes from a to 5, and
y where it passes from 6 to a, the electromotive force will be
F = Pax-Pbx + Qbx~ Qby + Pby-Pav + Qay- Qat,
where Pax signifies the value of P for the metal a at the tempera
ture #, or
Since in unequally heated circuits of different metals there are in
304 CONTACT FORCE. [252.
general thermoelectric currents, it follows that P and Q are in
genera] different for the same metal and same temperature.
252.] The existence of the quantity Q was first demonstrated by
Sir W. Thomson, in the memoir we have referred to, as a deduction
from the phenomenon of thermoelectric inversion discovered by
Gumming *, who found that the order of certain metals in the ther
moelectric scale is different at high and at low temperatures, so that
for a certain temperature two metals may be neutral to each other.
Thus, in a circuit of copper and iron if one junction be kept at the
ordinary temperature while the temperature of the other is raised,
a current sets from copper to iron through the hot junction, and
the electromotive force continues to increase till the hot junction
has reached a temperature T, which, according to Thomson, is
about 284°C. When the temperature of the hot junction is raised
still further the electromotive force is reduced, and at last, if the
temperature be raised high enough, the current is reversed. The
reversal of the current may be obtained more easily by raising the
temperature of the colder junction. If the temperature of both
junctions is above T the current sets from iron to copper through
the hotter junction, that is, in the reverse direction to that ob
served when both junctions are below T.
Hence, if one of the junctions is at the neutral temperature T
and the other is either hotter or colder, the current will set from
copper to iron through the junction at the neutral temperature.
253.] From this fact Thomson reasoned as follows : —
Suppose the other junction at a temperature lower than T.
The current may be made to work an engine or to generate heat in
a wire, and this expenditure of energy must be kept up by the
transformation of heat into electric energy, that is to say, heat
must disappear somewhere in the circuit. Now at the tempera
ture T iron and copper are neutral to each other, so that no
reversible thermal effect is produced at the hot junction, and at
the cold junction there is, by Peltier's principle, an evolution of
heat. Hence the only place where the heat can disappear is in the
copper or iron portions of the circuit, so that either a current in
iron from hot to cold must cool the iron, or a current in copper
from cold to hot must cool the copper, or both these effects may
take place. By an elaborate series of ingenious experiments Thom
son succeeded in detecting the reversible thermal action of the
current in passing between parts of different temperatures, and
* Cambridge Transactions, 1823.
254-] EXPERIMENTS OF TAIT. 305
he found that the current produced opposite effects in copper and
in iron *.
When a stream of a material fluid passes along a tube from
a hot part to a cold part it heats the tube, and when it passes
from cold to hot it cools the tube, and these effects depend on
the specific capacity for heat of the fluid. If we supposed elec
tricity, whether positive or negative, to be a material fluid, we
might measure its specific heat by the thermal effect on an un
equally heated conductor. Now Thomson's experiments shew that
positive electricity in copper and negative electricity in iron carry
heat with them from hot to cold. Hence, if we supposed either
positive or negative electricity to be a fluid, capable of being
heated and cooled, and of communicating heat to other bodies, we
should find the supposition contradicted by iron f6r positive elec
tricity and by copper for negative electricity, so that we should
have to abandon both hypotheses.
This scientific prediction of the reversible effect of an electric
current upon an unequally heated conductor of one metal is another
instructive example of the application of the theory of Conservation
of Energy to indicate new directions of scientific research. Thomson
has also applied the Second Law of Thermodynamics to indicate
relations between the quantities which we have denoted by P
and Q, and has investigated the possible thermoelectric properties
of bodies whose structure is different in different directions. He
has also investigated experimentally the conditions under which
these properties are developed by pressure, magnetization, &c.
254.] Professor Taitf has recently investigated the electro
motive force of thermoelectric circuits of different metals, having
their junctions at different temperatures. He finds that the elec
tromotive force of a circuit may be expressed veiy accurately by
the formula
E= • (4-4) ft-* A +4)1
where ^ is the absolute temperature of the hot junction, t2 that
of the cold junction, and tQ the temperature at which the two metals
are neutral to each other. The factor a is a coefficient depending
on the nature of the two metals composing the circuit. This law
has been verified through considerable ranges of temperature by
Professor Tait and his students, and he hopes to make the thermo
electric circuit available as a thermometric instrument in his
* ' On the Electrodynamic Qualities of Metals.' Phil Tram., 1856.
t Proc. R. S. Edin., Session 18/0-71, p. 308, also Dec. 18, 1871.
VOL. I. X
306 CONTACT FORCE. [254.
experiments on the conduction of heat, and in other cases in which
the mercurial thermometer is not convenient or has not a sufficient
range.
According to Tait's theory, the quantity which Thomson calls
the specific heat of electricity is proportional to the absolute tem
perature in each pure metal, though its magnitude and even its
sign vary in different metals. From this he has deduced by ther-
modynamic principles the following results. Let hat, 7cbt, kct
be the specific heats of electricity in three metals a, b, c, and let
Tbc, Tca, Tab be the temperatures at which pairs of these metals are
neutral to each other, then the equations
kb}Tab = 0,
express the relation of the neutral temperatures, the value of the
Peltier effect, and the electromotive force of a thermoelectric circuit.
CHAPTER IV.
ELECTROLYSIS.
Electrolytic Conduction.
255.] I HAVE already stated that when an electric current in
any part of its circuit passes through certain compound substances
called Electrolytes, the passage of the current is accompanied by
a certain chemical process called Electrolysis, in which the substance
is resolved into two components called Ions, of which one, called
the Anion, or the electronegative component, appears at the Anode,
or place where the current enters the electrolyte, and the other,
called the Cation, appears at the Cathode, or the place where the
current leaves the electrolyte.
The complete investigation of Electrolysis belongs quite as much
to Chemistry as to Electricity. We shall consider it from an
electrical point of view, without discussing its application to the
theory of the constitution of chemical compounds.
Of all electrical phenomena electrolysis appears the most likely
to furnish us with a real insight into the true nature of the electric
current, because we find currents of ordinary matter and currents
of electricity forming essential parts of the same phenomenon.
It is probably for this very reason that, in the present imperfectly
formed state of our ideas about electricity, the theories of electro
lysis are so unsatisfactory.
The fundamental law of electrolysis, which was established by
Faraday, and confirmed by the experiments of Beetz, Hittorf, and
others down to the present time, is as follows : —
The number of electrochemical equivalents of an electrolyte which
are decomposed by the passage of an electric current during a given
time is equal to the number of units of electricity which are trans
ferred by the current in the same time.
The electrochemical equivalent of a substance is that quantity
X 2,
308 ELECTROLYSIS. [255.
of the substance which is electrolysed by a unit current passing
through the substance for a unit of time, or, in other words, by the
passage of a unit of electricity. When the unit of electricity is
denned in absolute measure the absolute value of the electro
chemical equivalent of each substance can be determined in grains
or in grammes.
The electrochemical equivalents of different substances are pro
portional to their ordinary chemical equivalents. The ordinary
chemical equivalents, however, are the mere numerical ratios in
which the substances combine, whereas the electrochemical equi
valents are quantities of matter of a determinate magnitude, de
pending on the definition of the unit of electricity.
Every electrolyte consists of two components, which, during the
electrolysis, appear where the current enters and leaves the elec
trolyte, and nowhere else. Hence, if we conceive a surface described
within the substance of the electrolyte, the amount of electrolysis
which takes place through this surface, as measured by the elec
trochemical equivalents of the components transferred across it
in opposite directions, will be proportional to the total electric
current through the surface.
The actual transfer of the ions through the substance of the
electrolyte in opposite directions is therefore part of the phenomenon
of the conduction of an electric current through an electrolyte. At
every point of the electrolyte through which an electric current
is passing there are also two opposite material currents of the anion
and the cation, which have the same lines of flow with the electric
current, and are proportional to it in magnitude.
It is therefore extremely natural to suppose that the currents of
the ions are convection currents of electricity, and, in particular,
that every molecule of the cation is charged with a certain fixed
quantity of positive electricity, which is the same for the molecules
of all cations, and that every molecule of the anion is charged with
an equal quantity of negative electricity.
The opposite motion of the ions through the electrolyte would
then be a complete physical representation of the electric current.
We may compare this motion of the ions with the motion of gases
and liquids through each other during the process of diffusion,
there being this difference between the two processes, that, in
diffusion, the different substances are only mixed together and the
mixture is not homogeneous, whereas in electrolysis they are chemi
cally combined and the electrolyte is homogeneous. In diffusion
257-] THEORY OP CLAUSIUS. 309
the determining cause of the motion of a substance in a given
direction is a diminution of the quantity of that substance per
unit of volume in that direction, whereas in electrolysis the motion
of each ion is due to the electromotive force acting on the charged
molecules.
256.] Clausius *, who has bestowed much study on the theory
of the molecular agitation of bodies, supposes that the molecules
of all bodies are in a state of constant agitation, but that in solid
bodies each molecule never passes beyond a certain distance from
its original position, whereas in fluids a molecule, after moving
a certain distance from its original position, is just as likely to
move still farther from it as to move back again. Hence the
molecules of a fluid apparently at rest are continually changing
their positions, and passing irregularly from one part of the fluid
to another. In a compound fluid he supposes that not only the
compound molecules travel about in this way, but that, in the
collisions which occur between the compound molecules, the mole
cules of which they are composed are often separated and change
partners, so that the same individual atom is at one time associated
with one atom of the opposite kind, and at another time with another.
This process Clausius supposes to go on in the liquid at all times, but
when an electromotive force acts on the liquid the motions of the
molecules, which before were indifferently in all directions, are now
influenced by the electromotive force, so that the positively charged
molecules have a greater tendency towards the cathode than towards
the anode, and the negatively charged molecules have a greater
tendency to move in the opposite direction. Hence the molecules
of the cation will during their intervals of freedom struggle towards
the cathode, but will continually be checked in their course by
pairing for a time with molecules of the anion, which are also
struggling through the crowd, but in the opposite direction.
257.] This theory of Clausius enables us to understand how it is,
that whereas the actual decomposition of an electrolyte requires an
electromotive force of finite magnitude, the conduction of the
current in the electrolyte obeys the law of Ohm, so that every
electromotive force within the electrolyte, even the feeblest, produces
a current of proportionate magnitude.
According to the theory of Clausius, the decomposition and
recomposition of the electrolyte is continually going on even when
there is no current, and the veiy feeblest electromotive force is
* Fogg. Ann. bd. ci. s. 338 (1857).
310 ELECTROLYSIS. [25$.
sufficient to give this process a certain degree of direction, and so
to produce the currents of the ions and the electric current, which
is part of the same phenomenon. Within the electrolyte, however,
the ions are never set free in finite quantity, and it is this liberation
of the ions which requires a finite electromotive force. At the
electrodes the ions accumulate, for the successive portions of the
ions, as they arrive at the electrodes, instead of finding molecules of
the opposite ion ready to combine with them, are forced into com
pany with molecules of their own kind, with which they cannot
combine. The electromotive force required to produce this effect
is of finite magnitude, and forms an opposing electromotive force
which produces a reversed current when other electromotive forces
are removed. When this reversed electromotive force, owing to
the accumulation of the ions at the electrode, is observed, the
electrodes are said to be Polarized.
258.] , One of the best methods of determining whether a body
is or is not an electrolyte is to place it between platinum electrodes
and to pass a current through it for some time, and then, dis
engaging the electrodes from the voltaic battery, and connecting
them with a galvanometer, to observe whether a reverse current,
due to polarization of the electrodes, passes through the galvano
meter. Such a current, being due to accumulation of different
substances on the two electrodes, is a proof that the substance has
been electrolytically decomposed by the original current from the
battery. This method can often be applied where it is difficult,
by direct chemical methods, to detect the presence of the products
of decomposition at the electrodes. See Art. 271.
259.] So far as we have gone the theory of electrolysis appears
very satisfactory. It explains the electric current, the nature of
which we do not understand, by means of the currents of the
material components of the electrolyte, the motion of which,
though not visible to the eye, is easily demonstrated. It gives a
clear explanation, as Faraday has shewn, why an electrolyte which
conducts in the liquid state is a non-conductor when solidified, for
unless the molecules can pass from one part to another no elec
trolytic conduction can take place, so that the substance must
be in a liquid state, either by fusion or by solution, in order to be
a conductor.
But if we go on, and assume that the molecules of the ions
within the electrolyte are actually charged with certain definite
quantities of electricity, positive and negative, so that the elec-
260.] MOLECULAR CHARGE. 311
trolytic current is simply a current of convection, we find that this
tempting hypothesis leads us into very difficult ground.
In the first place, we must assume that in every electrolyte each
molecule of the cation, as it is liberated at the cathode, commu
nicates to the cathode a charge of positive electricity, the amount
of which is the same for every molecule, not only of that cation
but of all other cations. In the same way each molecule of the
anion when liberated, communicates to the anode a charge of
negative electricity, the numerical magnitude of which is the same
as that of the positive charge due to a molecule of a cation, but
with sign reversed.
If, instead of a single molecule, we consider an assemblage of
molecules, constituting an electrochemical equivalent of the ion,
then the total charge of all the molecules is, as we have seen, one
unit of electricity, positive or negative.
260.] We do not as yet know how many molecules there are
in an electrochemical equivalent of any substance, but the molecular
theory of chemistry, which is corroborated by many physical con
siderations, supposes that the number of molecules in an elec
trochemical equivalent is the same for all substances. We may
therefore, in molecular speculations, assume that the number of
molecules in an electrochemical equivalent is N, a number unknown
at present, but which we may hereafter find means to determine *.
Each molecule, therefore, on being liberated from the state of
combination, parts with a charge whose magnitude is -~ , and is
positive for the cation and negative for the anion. This definite
quantity of electricity we shall call the molecular charge. If it
were known it would be the most natural unit of electricity.
Hitherto we have only increased the precision of our ideas by
exercising our imagination in tracing the electrification of molecules
and the discharge of that electrification.
The liberation of the ions and the passage of positive electricity
from the anode and into the cathode are simultaneous facts. The
ions, when liberated, are not charged with electricity, hence, when
they are in combination, they have the molecular charges as above
described.
The electrification of a molecule, however, though easily spoken
of, is not so easily conceived.
We know that if two metals are brought into contact at any
* See note to Art. 5.
312 ELECTROLYSIS. [260.
point, the rest of their surfaces will be electrified, and if the metals
are in the form of two plates separated by a narrow interval of air,
the charge on each plate may become of considerable magnitude.
Something like this may be supposed to occur when the two
components of an electrolyte are in combination. Each pair of
molecules may be supposed to touch at one point, and to have the
rest of their surface charged with electricity due to the electro
motive force of contact.
But to explain the phenomenon, we ought to shew why the
charge thus produced on each molecule is of a fixed amount, and
why, when a molecule of chlorine is combined with a molecule of
zinc, the molecular charges are the same as when a molecule of
chlorine is combined with a molecule of copper, although the elec
tromotive force between chlorine and zinc is much greater than
that between chlorine and copper. If the charging of the molecules
is the effect of the electromotive force of contact, why should
electromotive forces of different intensities produce exactly equal
charges ?
Suppose, however, that we leap over this difficulty by simply
asserting the fact of the constant value of the molecular charge,
and that we call this constant molecular charge, for convenience in
description, one molecule of electricity.
This phrase, gross as it is, and out of harmony with the rest of
this treatise, will enable us at least to state clearly what is known
about electrolysis, and to appreciate the outstanding difficulties.
Every electrolyte must be considered as a binary compound of
its anion and its cation. The anion or the cation or both may be
compound bodies, so that a molecule of the anion or the cation
may be formed by a number of molecules of simple bodies. A
molecule of the anion and a molecule of the cation combined to
gether form one molecule of the electrolyte.
In order to act as an anion in an electrolyte, the molecule which
so acts must be charged with what we have called one molecule
of negative electricity, and in order to act as a cation the molecule
must be charged with one molecule of positive electricity.
These charges are connected with the molecules only when they
are combined as anion and cation in the electrolyte.
When the molecules are electrolysed, they part with their charges
to the electrodes, and appear as unelectrified bodies when set free
from combination.
If the same molecule is capable of acting as a cation in one
26 1.] SECONDARY PRODUCTS OF ELECTROLYSIS. 313
electrolyte and as an anion in another, and also of entering into
compound bodies which are not electrolytes, then we must suppose
that it receives a positive charge of electricity when it acts as a
cation, a negative charge when it acts as an anion, and that it
is without charge when it is not in an electrolyte.
Iodine, for instance, acts as an anion in the iodides of the metals
and in hydriodic acid, but is said to act as a cation in the bromide
of iodine.
This theory of molecular charges may serve as a method by
which we may remember a good many facts about electrolysis.
It is extremely improbable that when we come to understand the
true nature of electrolysis we shall retain in any form the theory of
molecular charges, for then we shall have obtained a secure basis
on which to form a true theory of electric currents, and so become
independent of these provisional theories.
261.] One of the most important steps in our knowledge of
electrolysis has been the recognition of the secondary chemical
processes which arise from the evolution of the ions at the elec
trodes.
In many cases the substances which are found at the electrodes
are not the actual ions of the electrolysis, but the products of the
action of these ions on the electrolyte.
Thus, when a solution of sulphate of soda is electrolysed by a
current which also passes through dilute sulphuric acid, equal
quantities of oxygen are given off at the anodes, and equal quan
tities of hydrogen at the cathodes, both in the sulphate of soda
and in the dilute acid.
But if the electrolysis is conducted in suitable vessels, such as
U-shaped tubes or vessels with a porous diaphragm, so that the
substance surrounding each electrode can be examined separately,
it is found that at the anode of the sulphate of soda there is an
equivalent of sulphuric acid as well as an equivalent of oxygen,
and at the cathode there is an equivalent of soda as well as two
equivalents of hydrogen.
It would at first sight seem as if, according to the old theory
of the constitution of salts, the sulphate of soda were electrolysed
into its constituents sulphuric acid and soda, while the water of the
solution is electrolysed at the same time into oxygen and hydrogen.
But this explanation would involve the admission that the same
current which passing through dilute sulphuric acid electrolyses
one equivalent of water, when it passes through solution of sulphate
314 ELECTROLYSIS. [261.
of soda electrolyses one equivalent of the salt as well as one equi
valent of the water, and this would be contrary to the law of
electrochemical equivalents.
But if we suppose that the components of sulphate of soda are
not SO3 and NaO but SO4 and Na, — not sulphuric acid and soda
but sulphion and sodium — then the sulphion travels to the anode
and is set free, but being unable to exist in a free state it breaks
up into sulphuric acid and oxygen, one equivalent of each. At
the same time the sodium is set free at the cathode, and there
decomposes the water of the solution, forming one equivalent of
soda and two of hydrogen.
In the dilute sulphuric acid the gases collected at the electrodes
are the constituents of water, namely one volume of oxygen and
two volumes of hydrogen. There is also an increase of sulphuric
acid at the anode, but its amount is not equal to an equivalent.
It is doubtful whether pure water is an electrolyte or not. The
greater the purity of the water., the greater the resistance to elec
trolytic conduction. The minutest traces of foreign matter are
sufficient to produce a great diminution of the electrical resistance
of water. The electric resistance of water as determined by different
observers has values so different that we cannot consider it as a
determined quantity. The purer the water the greater its resistance,
and if we could obtain really pure water it is doubtful whether it
would conduct at all.
As long as water was considered an electrolyte, and was, indeed,
taken as the type of electrolytes, there was a strong reason for
maintaining that it is a binary compound, and that two volumes
of hydrogen are chemically equivalent to one volume of oxygen.
If, however, we admit that water is not an electrolyte, we are free
to suppose that equal volumes of oxygen and of* hydrogen are
chemically equivalent.
The dynamical theory of gases leads us to suppose that in perfect
gases equal volumes always contain an equal number of molecules,
and that the principal part of the specific heat, that, namely, which
depends on the motion of agitation of the molecules among each
other, is the same for equal numbers of molecules of all gases.
Hence we are led to prefer a chemical system in which equal
volumes of oxygen and of hydrogen are regarded as equivalent,
and in which water is regarded as a compound of two equivalents
of hydrogen and one of oxygen, and therefore probably not capable
of direct electrolysis.
262.] DYNAMICAL THEORY. 315
While electrolysis fully establishes the close relationship between
electrical phenomena and those of chemical combination, the fact
that every chemical compound is not an electrolyte shews that
chemical combination is a process of a higher order of complexity
than any purely electrical phenomenon. Thus the combinations of
the metals with each other, though they are good conductors, and
their components stand at different points of the scale of electri
fication by contact, are not, even when in a fluid state,, decomposed
by the current. Most of the combinations of the substances which
act as anions are not conductors, and therefore are not electrolytes.
Besides these we have many compounds, containing the same com
ponents as electrolytes, but not in equivalent proportions, and these
are also non-conductors, and therefore not electrolytes.
On the Conservation of Energy in Electrolysis.
262.] Consider any voltaic circuit consisting partly of a battery,
partly of a wire, and partly of an electrolytic cell.
During the passage of unit of electricity through any section of
the circuit, one electrochemical equivalent of each of the substances
in the cells, whether voltaic or electrolytic, is electrolysed.
The amount of mechanical energy equivalent to any given
chemical process can be ascertained by converting the whole energy
due to the process into heat, and then expressing the heat in
dynamical measure by multiplying the number of thermal units by
Joule's mechanical equivalent of heat.
Where this direct method is not applicable, if we can estimate
the heat given out by the substances taken first in the state before
the process and then in the state after the process during their
reduction to a final state, which is the same in both cases, then the
thermal equivalent of the process is the difference of the two quan
tities of heat.
In the case in which the chemical action maintains a voltaic
circuit, Joule found that the heat developed in the voltaic cells is
less than that due to the chemical process within the cell, and that
the remainder of the heat is developed in the connecting wire, or,
when there is an electromagnetic engine in the circuit, part of the
heat may be accounted for by the mechanical work of the engine.
For instance, if the electrodes of the voltaic cell are first con
nected by a short thick wire, and afterwards by a long thin wire,
the heat developed in the cell for each grain of zinc dissolved is
greater in the first case than the second, but the heat developed
316 ELECTROLYSIS.
in the wire is greater in the second case than in the first. The
sum of the heat developed in the cell and in the wire for each grain
of zinc dissolved is the same in both cases. This has been esta
blished by Joule by direct experiment.
The ratio of the heat generated in the cell to that generated
in the wire is that of the resistance of the cell to that of the wire,
so that if the wire were made of sufficient resistance nearly the
whole of the heat would be generated in the wire, and if it were
made of sufficient conducting power nearly the whole of the heat
woula be generated in the cell.
Let the wire be made so as to have great resistance, then the
heat generated in it is equal in dynamical measure to the product
of the quantity of electricity which is transmitted, multiplied by
the electromotive force under which it is made to pass through
the wire.
263.] Now during the time in which an electrochemical equi
valent of the substance in the cell undergoes the chemical process
which gives rise to the current, one unit of electricity passes
through the wire. Hence, the heat developed by the passage
of one unit of electricity is in this case measured by the electro
motive force. But this heat is that which one electrochemical
equivalent of the substance generates, whether in the cell or in the
wire, while undergoing the given chemical process.
Hence the following important theorem, first proved by Thomson
(Phil. Mag. Dec. 1851):—
' The electromotive force of an electrochemical apparatus is in
absolute measure equal to the mechanical equivalent of the chemical
action on one electrochemical equivalent of the substance/
The thermal equivalents of many chemical actions have been
determined by Andrews, Hess, Favre and Silbermann, &c., and from
these their mechanical equivalents can be deduced by multiplication
by the mechanical equivalent of heat.
This theorem not only enables us to calculate from purely thermal
data the electromotive force of different voltaic arrangements, and
the electromotive force required to effect electrolysis in different
cases, but affords the means of actually measuring chemical affinity.
It has long been known that chemical affinity, or the tendency
which exists towards the going on of a certain chemical change,
is stronger in some cases than in others, but no proper measure
of this tendency could be made till it was shewn that this tendency
in certain cases is exactly equivalent to a certain electromotive
263-] CALCULATION OF ELECTROMOTIVE FORCE. 317
force, and can therefore be measured according to the very same
principles used in the measurement of electromotive forces.
Chemical affinity being therefore, in certain cases, reduced to
the form of a measurable quantity, the whole theory of chemical
processes, of the rate at which they go on, of the displacement of
one substance by another, &c., becomes much more intelligible than
when chemical affinity was regarded as a quality sui generis, and
irreducible to numerical measurement.
When the volume of the products of electrolysis is greater than
that of the electrolyte, work is done during the electrolysis in
overcoming the pressure. If the volume of an electrochemical
equivalent of the electrolyte is increased by a volume v when
electrolysed under a pressure p, then the work done during the
passage of a unit of electricity in overcoming pressure is v p, and
the electromotive force required for electrolysis must include a
part equal to vpt which is spent in performing this mechanical
work.
If the products of electrolysis are gases which, like oxygen and
hydrogen, are much rarer than the electrolyte, and fulfil Boyle's
law very exactly, vjo will be very nearly constant for the same
temperature, and the electromotive force required for electrolysis
will not depend in any sensible degree on the pressure. Hetfce it
has been found impossible to check the electrolytic decomposition
of dilute sulphuric acid by confining the decomposed gases in a
small space.
When the products of electrolysis are liquid or solid the quantity
vp will increase as the pressure increases, so that if v is positive
an increase of pressure will increase the electromotive force required
for electrolysis.
In the same way, any other kind of work done during electro
lysis will have an effect on the value of the electromotive force,
as, for instance, if a vertical current passes between two zinc
electrodes in a solution of sulphate of zinc a greater electromotive
force will be required when the current in the solution flows
upwards than when it flows downwards, for, in the first case, it
carries zinc from the lower to the upper electrode, and in the
second from the upper to the lower. The electromotive force
required for this purpose is less than the millionth part of that
of a Daniell's cell per foot.
CHAPTER V.
ELECTROLYTIC POLARIZATION.
264.] WHEN an electric current is passed through an electrolyte
bounded by metal electrodes, the accumulation of the ions at the
electrodes produces the phenomenon called Polarization, which
consists in an electromotive force acting in the opposite direction
to the current, and producing an apparent increase of the resistance.
When a continuous current is employed, the resistance appears
to increase rapidly from the commencement of the current, and
at last reaches a value nearly constant. If the form of the vessel
in which the electrolyte is contained is changed, the resistance is
altered in the same way as a similar change of form of a metallic
conductor would alter its resistance, but an additional apparent
resistance, depending on the nature of the electrodes, has always
to be added to the true resistance of the electrolyte.
265.] These phenomena have led some to suppose that there is
a finite electromotive force required for a current to pass through
an electrolyte. It has been shewn, however, by the researches of
Lenz, Neumann, Beetz, Wiedemann*, Paalzowf, and recently by
those of MM. F. Kohlrausch and W. A. NippoldtJ, that the con
duction in the electrolyte itself obeys Ohm's Law with the same
precision as in metallic conductors, and that the apparent resistance
at the bounding surface of the electrolyte and the electrodes is
entirely due to polarization.
266.] The phenomenon called polarization manifests itself in
the case of a continuous current by a diminution in the current,
indicating a force opposed to the current. Resistance is also per
ceived as a force opposed to the current, but we can distinguish
* Galvanismus, bd. i. t Berlin Monatsbericht, July, 1868.
£ Pogg, Ann. bd. cxxxviii. s. 286 (October, 1869).
267.] DISTINGUISHED FROM RESISTANCE. 319
between the two phenomena by instantaneously removing or re
versing the electromotive force.
The resisting force is always opposite in direction to the current,
and the external electromotive force required to overcome it is
proportional to the strength of the current, and changes its direc
tion when the direction of the current is changed. If the external
electromotive force becomes zero the current simply stops.
The electromotive force due to polarization, on the other hand,
is in a fixed direction, opposed to the current which produced it.
If the electromotive force which produced the current is removed,
the polarization produces a current in the opposite direction.
The difference between the two phenomena may be compared
with the difference between forcing a current of water through
a long capillary tube, and forcing water through a tube of moderate
length up into a cistern. In the first case if we remove the pressure
which produces the flow the current will simply stop. In the
second case, if we remove the pressure the water will begin to flow
down again from the cistern.
To make the mechanical illustration more complete, we have only
to suppose that the cistern is of moderate depth, so that when a
certain amount of water is raised into it, it begins to overflow.
This will represent the fact that the total electromotive force due
to polarization has a maximum limit.
267.] The cause of polarization appears to be the existence at
the electrodes of the products of the electrolytic decomposition of
the fluid between them. The surfaces of the electrodes are thus
rendered electrically different, and an electromotive force between
them is called into action, the direction of which is opposite to that
of the current which caused the polarization.
The ions, which by their presence at the electrodes produce the
phenomena of polarization, are not in a perfectly free state, but
are in a condition in which they adhere to the surface of the
electrodes with considerable force.
The electromotive force due to polarization depends upon the
density with which the electrode is covered with the ion, but it
is not proportional to this density, for the electromotive force does
not increase so rapidly as this density.
This deposit of the ion is constantly tending to become free,
and either to diffuse into the liquid, to escape as a gas, or to be
precipitated as a solid.
The rate of this dissipation of the polarization is exceedingly
320 ELECTROLYTIC POLARIZATION. [268.
small for slight degrees of polarization, and exceedingly rapid near
the limiting value of polarization.
268.] We have seen, Art. 262, that the electromotive force acting
in any electrolytic process is numerically equal to the mechanical
equivalent of the result of that process on one electrochemical
equivalent of the substance. If the process involves a diminution
of the intrinsic energy of the substances which take part in it,
as in the voltaic cell, then the electromotive force is in the direction
of the current. If the process involves an increase of the intrinsic
energy of the substances, as in the case of the electrolytic cell,
the electromotive force is in the direction opposite to that of the
current, and this electromotive force is called polarization.
In the case of a steady current in which electrolysis goes on
continuously, and the ions are separated in a free state at the
electrodes, we have only by a suitable process to measure the
intrinsic energy of the separated ions, and compare it with that
of the electrolyte in order to calculate the electromotive force
required for the electrolysis. This will give the maximum polari
zation.
But during the first instants of the process of electrolysis the
ions when deposited at the electrodes are not in a free state, and
their intrinsic energy is less than their energy in a free state,
though greater than their energy when combined in the electrolyte.
In fact, the ion in contact with the electrode is in a state which
when the deposit is very thin may be compared with that of
chemical combination with the electrode, but as the deposit in
creases in density, the succeeding portions are no longer so in
timately combined with the electrode, but simply adhere to it, and
at last the deposit, if gaseous, escapes in bubbles, if liquid, diffuses
through the electrolyte, and if solid, forms a precipitate.
In studying polarization we have therefore to consider
(1) The superficial density of the deposit, which we may call
o-. This quantity <r represents the number of electrochemical
equivalents of the ion deposited on unit of area. Since each
electrochemical equivalent deposited corresponds to one unit of
electricity transmitted by the current, we may consider o- as re
presenting either a surface-density of matter or a surface-density of
electricity.
(2) The electromotive force of polarization, which we may call p.
This quantity p is the difference between the electric potentials
of the two electrodes when the current through the electrolyte
2/0.] DISSIPATION OF THE DEPOSIT. 321
is so feeble that the proper resistance of the electrolyte makes no
sensible difference between these potentials.
The electromotive force p at any instant is numerically equal
to the mechanical equivalent of the electrolytic process going on at
that instant which corresponds to one electrochemical equivalent of
the electrolyte. This electrolytic process, it must be remembered,
consists in the deposit of the ions on the electrodes, and the state
in which they are deposited depends on the actual state of the
surface of the electrodes, which may be modified by previous
deposits.
Hence the electromotive force at any instant depends on the
previous history of the electrode. It is, speaking very roughly,
a function of cr, the density of the deposit, such that p — 0 when
a = 0, but p approaches a limiting value much sooner than a- does.
The statement, however, that p is a function of cr cannot be
considered accurate. It would be more correct to say that p is
a function of the chemical state of the superficial layer of the
deposit, and that this state depends on the density of the deposit
according to some law involving the time.
269.] (3) The third thing we must take into account is the
dissipation of the polarization. The polarization when left to itself
diminishes at a rate depending partly on the intensity of the
polarization or the density of the deposit, and partly on the nature
of the surrounding medium, and the chemical, mechanical, or thermal
action to which the surface of the electrode is exposed.
If we determine a time T such that at the rate at which
the deposit is dissipated, the whole deposit would be removed in
a time T> we may call T the modulus of the time of dissipation.
When the density of the deposit is very small, T is very large,
and may be reckoned by days or months. When the density of
the deposit approaches its limiting value T diminishes very rapidly,
and is probably a minute fraction of a second. In fact, the rate
of dissipation increases so rapidly that when the strength of the
current is maintained constant, the separated gas, instead of con
tributing to increase the density of the deposit, escapes in bubbles
as fast as it is formed.
270.] There is therefore a great difference between the state of
polarization of the electrodes of an electrolytic cell when the polari
zation is feeble, and when it is at its maximum value. For instance,
if a number of electrolytic cells of dilute sulphuric acid with
platinum electrodes are arranged in series, and if a small electro-
VOL. i. y
322 ELECTROLYTIC POLARIZATION. [27J-
motive force, such as that of one Daniell's cell, be made to act
on the circuit, the electromotive force will produce a current of
exceedingly short duration, for after a very short time the elec
tromotive force arising from the polarization of the cell will balance
that of the Daniell's cell.
The dissipation will be very small in the case of so feeble a state
of polarization, and it will take place by a very slow absorption
of the gases and diffusion through the liquid. The rate of this
dissipation is indicated by the exceedingly feeble current which
still continues to flow without any visible separation of gases.
If we neglect this dissipation for the short time during which
the state of polarization is set up, and if we call Q the total
quantity of electricity which is transmitted by the current during
this time, then if A is the area of one of the electrodes, and a-
the density of the deposit, supposed uniform,
e = ^(r,
If we now disconnect the electrodes of the electrolytic apparatus
from the Daniell's cell, and connect them with a galvanometer
capable of measuring the whole discharge through it, a quantity
of electricity nearly equal to Q will be discharged as the polari
zation disappears.
271.] Hence we may compare the action of this apparatus, which
is a form of Ritter's Secondary Pile, with that of a Leyden jar.
Both the secondary pile and the Leyden jar are capable of being
charged with a certain amount of electricity, and of being after
wards discharged. During the discharge a quantity of electricity
nearly equal to the charge passes in the opposite direction. The
difference between the charge and the discharge arises partly from
dissipation, a process which in the case of small charges is very
slow, but which, when the charge exceeds a certain limit, becomes
exceedingly rapid. Another part of the difference between the charge
and the discharge arises from the fact that after the electrodes
have been connected for a time sufficient to produce an apparently
complete discharge, so that the current has completely disappeared,
if we separate the electrodes for a time, and afterwards connect
them, we obtain a second discharge in the same direction as the
original discharge. This is called the residual discharge, and is a
phenomenon of the Leyden jar as well as of the secondary pile.
The secondary pile may therefore be compared in several respects
to a Leyden jar. There are, however3 certain important differences.
The charge of a Leyden jar is very exactly proportional to the
271.] COMPARISON WITH LEYDEN JAR. 323
electromotive force of the charge, that is, to the difference of
potentials of the two surfaces, and the charge corresponding to unit
of electromotive force is called the capacity of the jar, a constant
quantity. The corresponding quantity, which may be called the
capacity of the secondary pile, increases when the electromotive
force increases.
The capacity of the jar depends on the area of the opposed
surfaces, on the distance between them, and on the nature of the
substance between them, but not on the nature of the metallic
surfaces themselves. The capacity of the secondary pile depends
on the area of the surfaces of the electrodes, but not on the distance
between them, and it depends on the nature of the surface of the
electrodes, as well as on that of the fluid between them. The
maximum difference of the potentials of the electrodes in each
element of a secondary pile is very small compared with the maxi
mum difference of the potentials of those of a charged Leyden jar,
so that in order to obtain much electromotive force a pile of many
elements must be used.
On the other hand, the superficial density of the charge in the
secondary pile is immensely greater than the utmost superficial
density of the charge which can be accumulated on the surfaces
of a Leyden jar, insomuch that Mr. C. F. Varley *, in describing
the construction of a condenser of great capacity, recommends a
series of gold or platinum plates immersed in dilute acid as prefer
able in point of cheapness to induction plates of tinfoil separated
by insulating material.
The form in which the energy of a Leyden jar is stored up
is the state of constraint of the dielectric between the conducting
surfaces, a state which I have already described under the name
of electric polarization, pointing out those phenomena attending
this state which are at present known, and indicating the im
perfect state of our knowledge of what really takes place. See
Arts. 62, 111.
The form in which the energy of the secondary pile is stored
up is the chemical condition of the material stratum at the surface
of the electrodes, consisting of the ions of the electrolyte and the
substance of the electrodes in a relation varying from chemical
combination to superficial condensation, mechanical adherence, or
simple juxtaposition.
The seat of this energy is close to the surfaces of the electrodes,
* Specification of C. F. Varley, • Electric Telegraphs, &c.,' Jan. 1860.
Y 2
324 ELECTROLYTIC POLARIZATION.
and not throughout the substance of the electrolyte, and the form
in which it exists may be called electrolytic polarization.
After studying- the secondary pile in connexion with the Leyden
jar, the student should again compare the voltaic battery with
some form of the electrical machine, such as that described in
Art. 211.
Mr. Varley has lately * found that the capacity of one square
inch is from 175 to 542 microfarads and upwards for platinum
plates in dilute sulphuric acid, and that the capacity increases with
the electromotive force, being about 175 for 0.02 of a Daniell's
cell, and 542 for 1.6 Daniell's cells.
But the comparison between the Leyden jar and the secondary
pile may be carried still farther, as in the following experiment,
due to Bufff. It is only when the glass of the jar is cold that
it is capable of retaining a charge. At a temperature below 100°C
the glass becomes a conductor. If a test-tube containing mercury
is placed in a vessel of mercury, and if a pair of electrodes are
connected, one with the inner and the other with the outer portion
of mercury, the arrangement constitutes a Leyden jar which will
hold a charge at ordinary temperatures. If the electrodes are con
nected with those of a voltaic battery, no current will pass as long
as the glass is cold, but if the apparatus is gradually heated a
current will begin to pass, and will increase rapidly in intensity as
the temperature rises, though the glass remains apparently as hard
as ever.
This current is manifestly electrolytic, for if the electrodes are
disconnected from the battery, and connected with a galvanometer,
a considerable reverse current passes, due to polarization of the
surfaces of the glass.
If, while the battery is in action the apparatus is cooled, the
current is stopped by the cold glass as before, but the polarization
of the surfaces remains. The mercury may be removed, the surfaces
may be washed with nitric acid and with water, and fresh mercury
introduced. If the apparatus is then heated, the current of polar
ization appears as soon as the glass is sufficiently warm to conduct it.
We may therefore regard glass at 100°C, though apparently a
solid body, as an electrolyte, and there is considerable reason
to believe that in most instances in which a dielectric has a
slight degree of conductivity the conduction is electrolytic. The
* Proc. R. 8., Jan. 12, 1871.
t Annalen der Chemie und Pharmacie, bd. xc. 257 (1854).
272.] CONSTANT VOLTAIC ELEMENTS. 325
existence of polarization may be regarded as conclusive evidence of
electrolysis, and if the conductivity of a substance increases as the
temperature rises, we have good grounds for suspecting that it is
electrolytic.
On Constant Voltaic Elements.
272.] When a series of experiments is made with a voltaic
battery in which polarization occurs, the polarization diminishes
during the time that the current is not flowing, so that when
it begins to flow again the current is stronger than after it has
flowed for some time. If, on the other hand, the resistance of the
circuit is diminished by allowing the current to flow through a
short shunt, then, when the current is again made to flow through
the ordinary circuit, it is at first weaker than its normal strength
on account of the great polarization produced by the use of the
short circuit.
To get rid of these irregularities in the current, which are
exceedingly troublesome in experiments involving exact measure
ments, it is necessary to get rid of the polarization, or at least
to reduce it as much as possible.
It does not appear that there is much polarization at the surface
of the zinc plate when immersed in a solution of sulphate of zinc
or in dilute sulphuric acid. The principal seat of polarization is
at the surface of the negative metal. When the fluid in which
the negative metal is immersed is dilute sulphuric acid, it is seen
to become covered with bubbles of hydrogen gas, arising from the
electrolytic decomposition of the fluid. Of course these bubbles,
by preventing the fluid from touching the metal, diminish the
surface of contact and increase the resistance of the circuit. But
besides the visible bubbles it is certain that there is a thin coating
of hydrogen, probably not in a free state, adhering to the metal,
and as we have seen that this coating is able to produce an elec
tromotive force in the reverse direction, it must necessarily diminish
the electromotive force of the battery.
Various plans have been adopted to get rid of this coating of
hydrogen. It may be diminished to some extent by mechanical
means, such as stirring the liquid, or rubbing the surface of the
negative plate. In Smee's battery the negative plates are vertical,
and covered with finely divided platinum from which the bubbles of
hydrogen easily escape, and in their ascent produce a current of
liquid which helps to brush off other bubbles as they are formed.
A far more efficacious method, however, is to employ chemical
326 ELECTROLYTIC POLARIZATION.
means. These are of two kinds. In the batteries of Grove and
Bunsen the negative plate is immersed in a fluid rich in oxygen,
and the hydrogen, instead of forming a coating on the plate,
combines with this substance. In Grove's battery the plate is
of platinum immersed in strong nitric acid. In Bunsen's first
battery it is of carbon in the same acid. Chromic acid is also used
for the same purpose, and has the advantage of being free from the
acid fumes produced by the reduction of nitric acid.
A different mode of getting rid of the hydrogen is by using
copper as the negative metal, and covering the surface with a coat
of oxide. This, however, rapidly disappears when it is used as
the negative electrode. To renew it Joule has proposed to make
the copper plates in the form of disks, half immersed in the liquid,
and to rotate them slowly, so that the air may act on the parts
exposed to it in turn.
The other method is by using as the liquid an electrolyte, the
cation of which is a metal highly negative to zinc.
In Daniell's battery a copper plate is immersed in a saturated
solution of sulphate of copper. When the current flows through
the solution from the zinc to the copper no hydrogen appears on
the copper plate, but copper is deposited on it. When the solution
is saturated, and the current is not too strong, the copper appears
to act as a true cation, the anion S O4 travelling towards the zinc.
When these conditions are not fulfilled hydrogen is evolved at
the cathode, but immediately acts on the solution, throwing down
copper, and uniting with S O4 to form oil of vitriol. When this
is the case, the sulphate of copper next the copper plate is replaced
by oil of vitriol, the liquid becomes colourless, arid polarization by
hydrogen gas again takes place. The copper deposited in this way
is of a looser and more friable structure than that deposited by true
electrolysis.
To ensure that the liquid in contact with the copper shall be
saturated with sulphate of copper, crystals of this substance must
be placed in the liquid close to the copper, so that when the solution
is made weak by the deposition of the copper, more of the crystals
may be dissolved.
We have seen that it is necessary that the liquid next the copper
should be saturated with sulphate of copper. It is still more
necessary that the liquid in which the zinc is immersed should be
free from sulphate of copper. If any of this salt makes its way
to the surface of the zinc it is reduced, and copper is deposited
272.]
THOMSON'S FORM OF DANIELL'S CELL.
327
on the zinc. The zinc, copper, and fluid then form a little circuit
in which rapid electrolytic action goes on, and the zinc is eaten
away by an action which contributes nothing to the useful effect
of the battery.
To prevent this, the zinc is immersed either in dilute sulphuric
acid or in a solution of sulphate of zinc, and to prevent the solution
of sulphate of copper from mixing with this liquid, the two liquids
are separated by a division consisting of bladder or porous earthen
ware, which allows electrolysis to take place through it, but
effectually prevents mixture of the fluids by visible currents.
In some batteries sawdust is used to prevent currents. The
experiments of Graham, however, shew that the process of diffusion
goes on nearly as rapidly when two liquids are separated by a
division of this kind as when they are in direct contact, provided
there are no visible currents, and it is probable that if a septum
is employed which diminishes the diffusion, it will increase in
exactly the same ratio the resistance of the element, because elec
trolytic conduction is a process the mathematical laws of which
have the same form as those of diffusion, and whatever interferes
with one must interfere equally with the other. The only differ
ence is that diffusion is always going on, while the current flows
only when the battery is in action.
In all forms of DanielFs battery the final result is that the
sulphate of copper finds its way to the zinc and spoils the battery.
To retard this result indefinitely, Sir W. Thomson * has constructed
Darnell's battery in the following form.
SIPHON-
ELECTRGDES
LEVEL Cf SIPHON
ZWSO+ 1 Cu SO*
COPPER
Fig. 21.
In each cell the copper plate is placed horizontally at the bottom
* Proc. R. 8., Jan. 19, 1871.
328 ELECTROLYTIC POLARIZATION. [272.
and a saturated solution of sulphate of zinc is poured over it. The
zinc is in the form of a grating and is placed horizontally near the
surface of the solution. A glass tube is placed vertically in the
solution with its lower end just above the surface of the copper
plate. Crystals of sulphate of copper are dropped down this tube,
and, dissolving in the liquid, form a solution of greater density
than that of sulphate of zinc alone, so that it cannot get to the
zinc except by diffusion. To retard this process of diffusion, a
siphon, consisting of a glass tube stuffed with cotton wick, is
placed with one extremity midway between the zinc and copper,
and the other in a vessel outside the cell, so that the liquid is
very slowly drawn off near the middle of its depth. To supply
its place, water, or a weak solution of sulphate of zinc, is added
above when required. In this way the greater part of the sulphate
of copper rising through the liquid by diffusion is drawn off by the
siphon before it reaches the zinc, and the zinc is surrounded by
liquid nearly free from sulphate of copper, and having a very slow
downward motion in the cell, which still further retards the upward
motion of the sulphate of copper. During the action of the battery
copper is deposited on the copper plate, and SO4 travels slowly
through the liquid to the zinc with which it combines, forming
sulphate of zinc. Thus the liquid at the bottom becomes less dense
by the deposition of the copper, and the liquid at the top becomes
more dense by the addition of the zinc. To prevent this action
from changing the order of density of the strata, and so producing
instability and visible currents in the vessel, care must be taken to
keep the tube well supplied with crystals of sulphate of copper,
and to feed the cell above with a solution of sulphate of zinc suffi
ciently dilute to be lighter than any other stratum of the liquid
in the cell.
Daniel! 's battery is by no means the most powerful in common
use. The electromotive force of Grove's cell is 192,000,000, of
DanielPs 107,900,000 and that of Bunsen's 188,000,000.
The resistance of Daniell's cell is in general greater than that of
Grove's or Bunsen's of the same size.
These defects, however, are more than counterbalanced in all
cases where exact measurements are required, by the fact that
Daniell's cell exceeds every other known arrangement in constancy
of electromotive force. It has also the advantage of continuing
in working order for a long time, and of emitting no gas.
CHAPTER VI.
LINEAR ELECTRIC CURRENTS.
Chi Systems of Linear Conductors.
273.] ANY conductor may be treated as a linear conductor if it
is arranged so that the current must always pass in the same manner
between two portions of its surface which are called its electrodes.
For instance, a mass of metal of any form the surface of which is
entirely covered with insulating material except at two places, at
which the exposed surface of the conductor is in metallic contact
with electrodes formed of a perfectly conducting material, may be
treated as a linear conductor. For if the current be made to enter
at one of these electrodes and escape at the other the lines of flow
will be determinate, and the relation between electromotive force,
current and resistance will be expressed by Ohm's Law, for the
current in every part of the mass will be a linear function of E.
But if there be more possible electrodes than two, the conductor
may have more than one independent current through it, and these
may not be conjugate to each other. See Art. 282.
Law.
274.] Let E be the electromotive force in a linear conductor
from the electrode Al to the electrode A.2. (See Art. 69.) Let
C be the strength of the electric current along the conductor, that
is to say, let C units of electricity pass across every section in
the direction A1 A.2 in unit of time, and let R be the resistance of
the conductor, then the expression of Ohm's Law is
E = CE. (1)
Linear Conductors arranged in Series.
275.] Let Alt A2 be the electrodes of the first conductor and let
the second conductor be placed with one of its electrodes in contact
330 LINEAR ELECTRIC CURRENTS. [276.
with A2t so that the second conductor has for its electrodes A2, A3.
The electrodes of the third conductor may be denoted by A3
and A4.
Let the electromotive force along each of these conductors be
denoted by JS12, E^ EM, and so on for the other conductors.
Let the resistance of the conductors be
Bl2> ^23 > ^34 > &C-
Then, since the conductors are arranged in series so that the same
current C flows through each, we have by Ohm's Law,
E12 = CR12, EM = CR^, EU = CR^. (2)
If E is the resultant electromotive force, and R the resultant
resistance of the system, we must have by Ohm's Law,
E = CR. (3)
NOW ^=^12 + ^3+ ^34» (4)
the sum of the separate electromotive forces,
= C (R12 -f 7^23 + 7£34) by equations (2).
Comparing this result with (3), we find
R = RU + RK + RU> (5)
Or, the resistance of a series of conductors is the sum of the resistances
of the conductors taken separately.
Potential at any Point of the Series.
Let A and C be the electrodes of the series, B a point between
them, a, c, and b the potentials of these points respectively. Let
Rl be the resistance of the part from A to JB, R2 that of the part
from B to C, and R that of the whole from A to C, then, since
a—b = R1C, l^c — R^C, and a—c — RC,
the potential at B is
which determines the potential at B when those at A and C are
given.
Resistance of a Multiple Conductor.
276.] Let a number of conductors ABZ, ACZ, ADZ be arranged
side by side with their extremities in contact with the same two
points A and Z. They are then said to be arranged in multiple
arc.
Let the resistances of these conductors be RI} R2, R% respect-
2/7-] SPECIFIC RESISTANCE AND CONDUCTIVITY. 331
ively, and the currents Clt C2, (73, and let the resistance of the
multiple conductor be R, and the total current C. Then, since the
potentials at A and Z are the same for all the conductors, they have
the same difference, which we may call E. We then have
E = C^ 2?! = (72 -^2 == ^3 ^3 = dl*
but C=Cl+C2+C9,
1111 ,-v
¥=^ + ^ + ^3-
Or, M£ reciprocal of the resistance of a multiple conductor is the sum
of the reciprocals of the component conductors.
If we call the reciprocal of the resistance of a conductor the
conductivity of the conductor, then we may say that the con
ductivity of a multiple conductor is the sum of the conductivities of
the component conductors.
Current in any Branch of a Multiple Conductor.
From the equations of the preceding article, it appears that if
(\ is the current in any branch of the multiple conductor, and
Rl the resistance of that branch,
4 = C£, ; ... ; (8)
where C is the total current, and R is the resistance of the multiple
conductor as previously determined.
Longitudinal Resistance of Conductors of Uniform Section.
277.] Let the resistance of a cube of a given material to a current
parallel to one of its edges be p, the side of the cube being unit of
length, p is called the ' specific resistance of that material for unit
of volume.'
Consider next a prismatic conductor of the same material whose
length is /, and whose section is unity. This is equivalent to I
cubes arranged in series. The resistance of the conductor is there
fore I p.
Finally, consider a conductor of length I and uniform section s.
This is equivalent to s conductors similar to the last arranged in
multiple arc. The resistance of this conductor is therefore
When we know the resistance of a uniform wire we can determine
332 LINEAR ELECTRIC CURRENTS. [^78.
the specific resistance of the material of which it is made if we can
measure its length and its section.
The sectional area of small wires is most accurately determined
by calculation from the length, weight, and specific gravity of the
specimen. The determination of the specific gravity is sometimes
inconvenient, and in such cases the resistance of a wire of unit
length and unit mass is used as the ' specific resistance per unit of
weight.'
If r is this resistance, I the length, and m the mass of a wire, then
*.*?.
m
On the Dimensions of the Quantities involved in these Equations.
278.] The resistance of a conductor is the ratio of the electro
motive force acting on it to the current produced. The conduct
ivity of the conductor is the reciprocal of this quantity, or in
other words, the ratio of the current to the electromotive force
producing it.
Now we know that in the electrostatic system of measurement
the ratio of a quantity of electricity to the potential of the con
ductor on which it is spread is the capacity of the conductor, and
is measured by a line. If the conductor is a sphere placed in an
unlimited field, this line is the radius of the sphere. The ratio
of a quantity of electricity to an electromotive force is therefore a
line, but the ratio of a quantity of electricity to a current is the
time during which the current flows to transmit that quantity.
Hence the ratio of a current to an electromotive force is that of a
line to a time, or in other words, it is a velocity.
The fact that the conductivity of a conductor is expressed in the
electrostatic system of measurement by a velocity may be verified
by supposing a sphere of radius r charged to potential V, and then
connected with the earth by the given conductor. Let the sphere
contract, so that as the electricity escapes through the conductor
the potential of the sphere is always kept equal to V. Then the
charge on the sphere is r V at any instant, and the current is
-j, :(rV\ but, since V is constant, the current is -IT ^i and the
electromotive force through the conductor is V.
The conductivity of the conductor is the ratio of the current to
the electromotive force, or -=- , that is, the velocity with which the
28O.] SYSTEM OF LINEAR CONDUCTORS. 333
radius of the sphere must diminish in order to maintain the potential
constant when the charge is allowed to pass to earth through the
conductor.
In the electrostatic system, therefore, the conductivity of a con
ductor is a velocity, and of the dimensions [LT'1'].
The resistance of the conductor is therefore of the dimensions
\L-*T\.
The specific resistance per unit of volume is of the dimension of
\T~\j and the specific conductivity per unit of volume is of the
dimension of [27"1]-
The numerical magnitude of these coefficients depends only on
the unit of time, which is the same in different countries.
The specific resistance per unit of weight is of the dimensions
279.] We shall afterwards find that in the electromagnetic
system of measurement the resistance of a conductor is expressed
by a velocity, so that in this system the dimensions of the resist
ance of a conductor are [ZI7"1].
The conductivity of the conductor is of course the reciprocal of
this.
The specific resistance per unit of volume in this system is of the
dimensions \_L2T~l'}) and the specific resistance per unit of weight
is of the dimensions \L~1T~1M].
On Linear Systems of Conductors in general.
280.] The most general case of a linear system is that of n
points , AH A.2, ... An, connected together in pairs by \n(n— 1)
linear conductors. Let the conductivity (or reciprocal of the re
sistance) of that conductor which connects any pair of points, say
Ap and Aq, be called Kpq, and let the current from Ap to Aq be Cpq.
Let j^ and Pq be the electric potentials at the points Ap and Aq
respectively, and let the internal electromotive force, if there be
any, along the conductor from Ap to Aq be Epq.
The current from Ap to Aq is, by Ohm's Law,
CM = KM(PP-P,+EM). (1)
Among these quantities we have the following sets of relations :
The conductivity of a conductor is the same in either direction,
or " KM = Kqp. (2)
The electromotive force and the current are directed quantities ,
so that Ept=-Eqp, and Cn=-Cv. (3)
334 LINEAR ELECTRIC CURRENTS. [280.
Let Plt P2,...Pn be the potentials at Alt A2, ... An respectively,
and let Qi> Q2, ... Qn be the quantities of electricity which enter
the system in unit of time at each of these points respectively.
These are necessarily subject to the condition of ' continuity'
Qi+Q*.-. +<?„= 0, (4)
since electricity can neither be indefinitely accumulated nor pro
duced within the system.
The condition of ' continuity ' at any point Ap is
QP = Cpl+Cp2 + &c. + Cpn. (5)
Substituting the values of the currents in terms of equation
(1), this becomes
Qp = (Kfl+Kft + toi.+KrJPf-(KAP1 + KrtPt + tos.+K,.PJ
+ (KpqEpl + &C.+Kp^fn). (G)
The symbol Kpp does not occur in this equation. Let us therefore
give it the value
£„ = - (*,i + K» + &c. + *,„) ; (7)
that is, let Kpp be a quantity equal and opposite to the sum of
all the conductivities of the conductors which meet in Ap. We
may then write the condition of continuity for the point Ap ,
flpn-Qp. (8)
By substituting 1, 2, &c. n for p in this equation we shall obtain
n equations of the same kind from which to determine the n
potentials Plf P2, &c., Pn.
Since, however, there is a necessary condition, (4), connecting the
values of Q, there will be only n—l independent equations. These
will be sufficient to determine the differences of the potentials of th e
points, but not to determine the absolute potential of any. This,
however, is not required to calculate the currents in the system.
If we denote by D the determinant
D =
(9)
and by Dpq, the minor of Km, we find for the value of Pp—
(Pp-Pn)D= (K1,S12+&o.-Ql)J)pl + (K21S.21 + &G.-Q2)Dl>2
+ (Kql Zql+&0. + Kqn Eqn- Qt) Dm + &c. (10)
In the same way the excess of the potential of any other point,
28 1.] SYSTEM OF LINEAR CONDUCTORS. 335
say Aq, over that of An may be determined. We may then de
termine the current between Ap and Aq from equation (1), and so
solve the problem completely.
281.] We shall now demonstrate a reciprocal property of any
two conductors of the system, answering to the reciprocal property
we have already demonstrated for statical electricity in Art. 88.
The coefficient of Qq in the expression for Pp is —~ . That of Qp
in the expression for Pq is —^ •
Now Dpq differs from Dqp only by the substitution of the symbols
such as Kqp for Kpq. But, by equation (2), these two symbols are
equal, since the conductivity of a conductor is the same both ways.
Hence Dpq = Dqp. (11)
It follows from this that the part of the potential at Ap arising
from the introduction of a unit current at Aq is equal to the part of
the potential at Aq arising from the introduction of a unit current
at Ap.
We may deduce from this a proposition of a more practical form.
Let A, £, C, D be any four points of the system, and let the
effect of a current Q, made to enter the system at A and leave it
at B, be to make the potential at C exceed that at D by P. Then,
if an equal current Q be made to enter the system at C and leave
it at D, the potential at A will exceed that at B by the same
quantity P.
We may also establish a property of a similar kind relating to
the effect of the internal electromotive force Ers) acting along the
conductor which joins the points Ar and As in producing an ex
ternal electromotive force on the conductor from Ap to Aq, that is
to say, a difference of potentials Pp — Pq. For since
%* = —£*>»
the part of the value of Pp which depends on this electromotive
force is 1
-p(Dpr-Dps)Ers,
and the part of the value of Pq is
~(Dqr-Dqa}Ert.
Therefore the coefficient of Ers in the value of Pp — Pq is
^-{Dp^Dq,-Dp,-Dqr}. (12)
This is identical with the coefficient of Epq in the value of Pr — P,.
336 LINEAR ELECTRIC CURRENTS. [282.
If therefore an electromotive force E be introduced, acting in the
conductor from A to B, and if this causes the potential at C to
exceed that at D by P, then the same electromotive force E intro
duced into the conductor from C to D will cause the potential at A
to exceed that at J5 by the same quantity P.
The electromotive force E may be that of a voltaic battery intro
duced between the points named, care being taken that the resist
ance of the conductor is the same before and after the introduction
of the battery.
282.] If Dfr+Dv-Dpt-Dqr = o, (13)
the conductor Ap Aq is said to be conjugate to ArA8, and we have
seen that this relation is reciprocal.
An electromotive force in one of two conjugate conductors pro
duces no electromotive force or current along the other. We shall
find the practical application of this principle in the case of the
electric bridge.
The theory of conjugate conductors has been investigated by
Kirchhoff, who has stated the conditions of a linear system in the
following manner, in which the consideration of the potential is
avoided.
(1) (Condition of ' continuity.') At any point of the system the
sum of all the currents which flow towards that point is zero.
(2) In any complete circuit formed by the conductors the sum
of the electromotive forces taken round the circuit is equal to the
sum of the products of the current in each conductor multiplied by
the resistance of that conductor.
We obtain this result by adding equations of the form (1) for the
complete circuit, when the potentials necessarily disappear.
Heat Generated in the System.
283.] The mechanical equivalent of the quantity of heat gene
rated in a conductor whose resistance is R by a current C in unit of
time is, by Art. 242, JH = £C2. (14)
We have therefore to determine the sum of such quantities as
RC2 for all the conductors of the system.
For the conductor from Ap to Aq the conductivity is Kpq, and the
resistance 2tpq, where K^ R^ = L (15)
The current in this conductor is, according to Ohm's Law,
CPq = Kpq(Pv-Pq). (16)
284.] GENERATION OF HEAT. 337
We shall suppose, however, that the value of the current is not
that given by Ohm's Law, but Xpq, where
To determine the heat generated in the system we have to find
the sum of all the quantities of the form
or JH=2{XrtC*M + 2SrtCr,Yrt + XMT*n}. (18)
Giving Cpq its value, and remembering the relation between Kpq
and flpq, this becomes
2(Pp-Pq)(Cpq + 2Yfq)+KfqY*M. (19)
Now since both C and X must satisfy the condition of continuity
at we have > (20)
qp = xpl+ XM+&C.+XW (21)
therefore 0 = Ypl + YP2 + &c. + Ypn . (22)
Adding together therefore all the terms of (19), we find
2(fi« JP«) = 2 P, ft, + S J^IV (23)
Now since R is always positive and Y2 is essentially positive, the
last term of this equation must be essentially positive. Hence the
first term is a minimum when Y is zero in every conductor, that is,
when the current in every conductor is that given by Ohm's Law.
Hence the following theorem :
284.] In any system of conductors in which there are no internal
electromotive forces the heat generated by currents distributed in
accordance with Ohm's Law is less than if the currents had been
distributed in any other manner consistent with the actual con
ditions of supply and outflow of the current.
The heat actually generated when Ohm's Law is fulfilled is
mechanically equivalent to 2 Pp Qq, that is, to the sum of the
products of the quantities of electricity supplied at the different
external electrodes, each multiplied by the potential at which it is
supplied.
VOL. i.
CHAPTER VII.
CONDUCTION IN THREE DIMENSIONS.
Notation of Electric Currents.
285.] AT any point let an element of area dS be taken normal
to the axis of #, and let Q units of electricity pass across this area
from the negative to the positive side in unit of time, then, if
J^ becomes ultimately equal to u when dSis indefinitely diminished,
u is said to be the Component of the electric current in the direction
of x at the given point.
In the same way we may determine v and w, the components of
the current in the directions of y and z respectively.
286.] To determine the component of the current in any other
direction OR through the given point 0.
Let /, m, n be the direction-cosines of OR, then cutting off from
the axes of &, y, z portions equal to
r r r
-j) — > and -
i m n
respectively at A, £ and (7, the triangle ABC
will be normal to OR.
The area of this triangle ABC will be
and by diminishing r this area may be diminished without limit.
The quantity of electricity which leaves the tetrahedron ABCO
by the triangle ABC must be equal to that which enters it through
the three triangles OBC, OCA, and OAB.
o
The area of the triangle OBC is £ — , and the component of
287.] COMPONENT AND RESULTANT CURRENTS. 339
the current normal to its plane is n, so that the quantity which
enters through this triangle is \ r2
mn
The quantities which enter through the triangles OCA and OAB
respectively are „ w
t ?* — ^ > and f r* -^ —
nl Im
If y is the component of the velocity in the direction OR, then
the quantity which leaves the tetrahedron through ABC is
Since this is equal to the quantity which enters through the three
other triangles,
ir'y = ir2$ u , v , w I.
2 Imn 2 \ mn nl Im J '
,,. , . , 2 Imn
multiplying by — ^ — > we get
y — lu + mv + nw. (1)
If we put n2 + v2 + w2 = F2,
and make /', m', n' such that
n = IT, v = m'r, and w = nT ;
then ysr^ + w^ + MwO- (2)
Hence, if we define the resultant current as a vector whose
magnitude is T, and whose direction-cosines are /', m'9 n', and if
y denotes the current resolved in a direction making an angle 0
with that of the resultant current, then
y = T cos 6 ; (3)
shewing that the law of resolution of currents is the same as that
of velocities, forces, and all other vectors.
287.] To determine the condition that a given surface may
be a surface of flow.
Let F(x,y,z) = \ (4)
be the equation of a family of surfaces any one of which is given by
making X constant, then, if we make
T~
dx
~J~ • ~J~
dy\ dz\
,-x
^ '
the direction-cosines of the normal, reckoned in the direction in
which X increases, are
-.-r^A. TIT^A TIT^A. /Ax
l = N--, m^N--i n = N--' (6)
Z 2
340
CONDUCTION IN THREE DIMENSIONS.
[288.
Hence, if y is the component of the current normal to the surface,
-*{
dX dX
u -=- 4- v -j-
dx dy
w
dX\
dz]
(7)
If y = 0 there will be no current through the surface, and the
surface may be called a Surface of Flow, because the lines of motion
are in the surface.
288.] The equation of a surface of flow is therefore
^A dX dX ,ftX
u - +v-j- +w -j- = 0. (8)
ax ay dz
If this equation is true for all values of A, all the surfaces of the
family will be surfaces of flow.
289.] Let there be another family of surfaces, whose parameter
is A', then, if these are also surfaces of flow, we shall have
dX' dA.' dX'
-j- +
dx
f- w
=
dz
(9)
If there is a third family of surfaces of flow, whose parameter
is A", then dK,,
u
dx
M
7
\-w
dz
= 0.
(10)
Eliminating between these three equations, u, v, and w disappear
together, and we find
dX
dx
dX'
dx
dX"
dX
dX'
dX"
= 0
or
(11)
(12)
dX
~dz
dX'
dz
dX"
v dy dz
X" = </> (A, A') ;
that is, X" is some function of A and X'.
290.] Now consider the four surfaces whose parameters are A,
A 4- 8 A, X', and A' + 5 A'. These four surfaces enclose a quadrilateral
tube, which we may call the tube 8A.8A'. Since this tube is
bounded by surfaces across which there is no flow, we may call
it a Tube of Flow. If we take any two sections across the tube,
the quantity which enters the tube at one section must be equal
to the quantity which leaves it at the other, and since this quantity
is therefore the same for every section of the tube, let us call it
I/bX.bX' where L is a function of A and A', the parameters which
determine the particular tube.
293-] TUBES OF FLOW. 341
291.] If bS denotes the section of a tube of flow by a plane
normal to a?, we have by the theory of the change of the inde
pendent variables,
*x *x' *<^7A^/A' ^^\ ns\
oA.oA. = oM — — — r--)i (I*)
\y dz dz ay '
and by the definition of the components of the current
\'. (14)
(15)
„. fdK d\ d\dX^ ~\
Hence u = L(-r —7 ~r -j- ) •
v dy dz dz dy *
,.-..,, T /d\ d\.' d\ d\\
similarly v = L ( -= — = = — 7- ) 3
v#2 aa? fite dz'
-j- /(t A.wA. ^ZA^/A\
\flnf. flu flu flnr>. /
V# 6^ dy dx
292.] It is always possible when one of the functions A or X' is
known, to determine the other so that L may be equal to unity.
For instance, let us take the plane of yz, and draw upon it a series
of equidistant lines parallel to y, to represent the sections of the
family A.' by this plane. In other words, let the function X' be
determined by the condition that when x = 0 A7= z. If we then
make L = 1, and therefore (when x = 0)
X =
then in the plane (x =• 0) the amount of electricity which passes
through any portion will be
/• r
(16)
Having determined the nature of the sections of the surfaces of
flow by the plane of yzy the form of the surfaces elsewhere is
determined by the conditions (8) and (9). The two functions A
and Ar thus determined are sufficient to determine the current at
every point by equations (15), unity being substituted for L.
On Lines of Flow.
293.] Let a series of values of A and of A' be chosen, the suc
cessive differences in each series being unity. The two series of
surfaces denned by these values will divide space into a system
of quadrilateral tubes through each of which there will be a unit
current. By assuming the unit sufficiently small, the details of
the current may be expressed by these tubes with any desired
amount of minuteness. Then if any surface be drawn cutting the
342 CONDUCTION IN THREE DIMENSIONS. [294-
system of tubes, the quantity of the current which passes through
this surface will be expressed by the number of tubes which cut it,
since each tube carries unity of current.
The actual intersections of the surfaces may be called Lines of
Flow. When the unit is taken sufficiently small, the number of
lines of flow which cut a surface is approximately equal to the
number of tubes of flow which cut it, so that we may consider
the lines of flow as expressing- not only the direction of the current
but its strength^ since each line of flow through a given section
corresponds to a unit current.
On Current-Sheets and Current-Functions.
294.] A stratum of a conductor contained between two con
secutive surfaces of flow of one system, say that of A', is called
a Current-Sheet. The tubes of flow within this sheet are deter
mined by the function A. If A^ and AP denote the values of A at
the points A and P respectively, then the current from right to
left across any line drawn on the sheet from A to P is AP — A^.
If AP be an element, ds, of a curve drawn on the sheet, the current
which crosses this element from right to left is
•d\ ,
-j-ds.
ds
This function A, from which the distribution of the current in
the sheet can be completely determined, is called the Current-
Function.
Any thin sheet of metal or conducting matter bounded on both
sides by air or some other non-conducting medium may be treated
as a current-sheet, in which the distribution of the current may
be expressed by means of a current-function. See Art. 647.
lion of ' Continuity.''
295.] If we differentiate the three equations (15) with respect to
x, y, z respectively, remembering that L is a function of A and A',
we find du dv dw
The corresponding equation in Hydrodynamics is called the
Equation of ' Continuity.' The continuity which it expresses is
the continuity of existence, that is, the fact that a material sub
stance cannot leave one part of space and arrive at another, without
going through the space between. It cannot simply vanish in the
295-] EQUATION OF CONTINUITY. 343
one place and appear in the other, but it must travel along a con
tinuous path, so that if a closed surface be drawn, including the
one place and excluding the other, a material substance in passing
from the one place to the other must go through the closed surface.
The most general form of the equation in hydrodynamics is
d(pu) d(pv) d(pw) dp , }
~fa~ ~Ji ~W ^Tt-
where p signifies the ratio of the quantity of the substance to the
volume it occupies, that volume being in this case the differential
element of volume, and (pu}} (pv), and (pw) signify the ratio of the
quantity of the substance which crosses an element of area in unit
of time to that area, these areas being normal to the axes of #, ^, and
z respectively. Thus understood, the equation is applicable to any
material substance, solid or fluid, whether the motion be continuous
or discontinuous, provided the existence of the parts of that sub
stance is continuous. If anything, though not a substance, is
subject to the condition of continuous existence in time and space,
the equation will express this condition. In other parts of Physical
Science, as, for instance, in the theory of electric and magnetic
quantities, equations of a similar form occur. We shall call such
equations ' equations of continuity' to indicate their form, though
we may not attribute to these quantities the properties of matter,
or even continuous existence in time and space.
The equation (17), which we have arrived at in the case of
electric currents, is identical with (18) if we make p = 1, that is,
if we suppose the substance homogeneous and incompressible. The
equation, in the case of fluids, may also be established by either
of the modes of proof given in treatises on Hydrodynamics. In
one of these we trace the course and the deformation of a certain
element of the fluid as it moves along. In the other, we fix our
attention on an element of space, and take account of all that
enters or leaves it. The former of these methods cannot be applied
to electric currents, as we do not know the velocity with which the
electricity passes through the body, or even whether it moves in
the positive or the negative direction of the current. All that we
know is the algebraical value of the quantity which crosses unit
of area in unit of time, a quantity corresponding to (pu) in the
equation (18). We have no means of ascertaining the value of
either of the factors p or u, and therefore we cannot follow a par
ticular portion of electricity in its course through the body. The
other method of investigation, in which we consider what passes
344 CONDUCTION IN THREE DIMENSIONS. [296.
through the walls of an element of volume, is applicable to electric
currents, and is perhaps preferable in point of form to that which
we have given, but as it may be found in any treatise on Hydro
dynamics we need not repeat it here.
Quantity of Electricity which passes through a given Surface.
296.] Let F be the resultant current at any point of the surface.
Let dS be an element of the surface, and let e be the angle between
F and the normal to the surface, then the total current through
the surface will be r r
J T cos e dS,
the integration being extended over the surface.
As in Art. 21, we may transform this integral into the form
in the case of any closed surface, the limits of the triple integration
being those included by the surface. This is the expression for
the total efflux from the closed surface. Since in all cases of steady
currents this must be zero whatever the limits of the integration,
the quantity under the integral sign must vanish, and we obtain
in this way the equation of continuity (17).
CHAPTER VIII.
RESISTANCE AND CONDUCTIVITY IN THREE DIMENSIONS.
On tke most General Relations betiveen Current and Electro
motive Force.
297.] LET the components of the current at any point be u, v, w.
Let the components of the electromotive force be X, Y, Z.
The electromotive force at any point is the resultant force oil
a unit of positive electricity placed at that point. It may arise
(1) from electrostatic action, in which case if V is the potential,
AV ar dr. m
X=~te' Yz ~Ty' -Tz'
or (2) from electromagnetic induction, the laws of which we shall
afterwards examine ; or (3) from thermoelectric or electrochemical
action at the point itself, tending to produce a current in a given
direction.
We shall in general suppose that X, F, Z represent the com
ponents of the actual electromotive force at the point, whatever
be the origin of the force, but we shall occasionally examine the
result of supposing it entirely due to variation of potential.
By Ohm's Law the current is proportional to the electromotive
force. Hence X, Y, Z must be linear functions of u, v, w. We
may therefore assume as the equations of Resistance,
X= ^u+Q3v + P2w^
Y = P^ + ^v + Q^^ (2)
Z —
We may call the coefficients R the coefficients of longitudinal
resistance in the directions of the axes of coordinates.
The coefficients P and Q may be called the coefficients of trans
verse resistance. They indicate the electromotive force in one
direction required to produce a current in a different direction.
346 RESISTANCE AND CONDUCTIVITY. [298.
If we were at liberty to assume that a solid body may be treated
as a system of linear conductors, then, from the reciprocal property
(Art. 281) of any two conductors of a linear system, we might shew
that the electromotive force along z required to produce a unit current
parallel to y must be equal to the electromotive force along y re
quired to produce a unit current parallel to z. This would shew
that P1= Qly and similarly we should find P2 = Q2, and P3 = Q3.
When these conditions are satisfied the system of coefficients is said
to be Symmetrical. When they are not satisfied it is called a
Skew system.
We have great reason to believe that in every actual case the
system is symmetrical, but we shall examine some of the con
sequences of admitting the possibility of a skew system.
298.] The quantities u, v, w may be expressed as linear functions
of X, Y, Z by a system of equations, which we may call Equations
of Conductivity,
u — r1X + #3Y+ q2Z,
v =toX+rtY+PlZ, (3)
w = X
we may call the coefficients r the coefficients of Longitudinal con
ductivity, andjfl and q those of Transverse conductivity.
The coefficients of resistance are inverse to those of conductivity.
This relation may be defined as follows :
Let [PQR] be the determinant of the coefficients of resistance,
and [pqr] that of the coefficients of conductivity, then
P1P2P,+ Q1Q2Q3+R1R2RB-P1Q1R1-P2Q2R2-P3Q3E3) (4)
[pqr] = PiPA + qiq<iqz + rlr2rs-plqlrl-p2q2rz-p3q3r^ (5)
[PQR] [pqr] = 1, (6)
\_PQK\p, = (P2P3- «i A), [pqr] P1 = (ptPz-q, r,l (7)
&c. &c.
The other equations may be formed by altering the symbols
P, Q, R, p, q, r, and the suffixes 1, 2, 3 in cyclical order.
Rate of Generation of Heat.
299.] To find the work done by the current in unit of time
in overcoming resistance, and so generating heat, we multiply the.
components of the current by the corresponding components of the
electromotive force. We thus obtain the following expressions for
W, the quantity of work expended in unit of time :
3OO.] COEFFICIENTS OF CONDUCTIVITY. 347
= S&2 + R.2v2 +R3w2+ (Pi + <3i) vw + (P2 + Qa) ww + (P3
uv '
By a proper choice of axes, either of the two latter equations may
be deprived of the terms involving the products of u, v, w or of
X, Yj Z. The system of axes, however, which reduces W to the form
is not in general the same as that which reduces it to the form
It is only when the coefficients P15 P2, P3 are equal respectively
to Q19 Q2, <23 that the two systems of axes coincide.
If with Thomson * we write
and p — -s + t,
then we have
= S-Tn
= s — t; )
and [PQR] r, = R.2R3-S^+T^, x
-f^^-^^, (13)
If therefore we cause Slf S2, S3 to disappear, ^ will not also dis
appear unless the coefficients T are zero.
Condition of Stability.
300.] Since the equilibrium of electricity is stable, the work
spent in maintaining the current must always be positive. The
conditions that W must be positive are that the three coefficients
R19 R2, RV and the three expressions
must all be positive.
There are similar conditions for the coefficients of conductivity.
* Tram. R. S. Edin., 1853-4, p. 165.
348 RESISTANCE AND CONDUCTIVITY. [3O1.
Equation of Continuity in a Homogeneous Medium.
301.] If we express the components of the electromotive force
as the derivatives of the potential F, the equation of continuity
du dv div
-T- + ~r + -r = 0
ax ay az
becomes in a homogeneous medium
d2F d2F d27 d27 d2F d27
<i -T-o- 2-r-5- -T5- <iT— T 2^~j- 3^~r
1 das2 2 dy2 dz2 1 dy dz L dzdx das ay
If the medium is not homogeneous there will be terms arising
from the variation of the coefficients of conductivity in passing
from one point to another.
This equation corresponds to Laplace's equation in an isotropic
medium.
302.] If we put
\rs\ = T! r2 r3+ 2 sl s2 ^-^ s^ -r2 s22 -r3 532, (17)
and [AS] = A1A2A3+2£13233-A1$l*-A2£22-A3B32, (18)
where \rs] At = r2r3 —
(19)
and so on, the system A, B will be inverse to the system /•, s, and
if we make
Alx2^A2y2^-A^z^ + 2Blyz + 2Bzzx-}-2Bzxy = \AS\ p2, (20)
we shall find that
F=^i (21)
4 77 p
is a solution of the equation.
In the case in which the coefficients T are zero, the coefficients A
and B become identical with R and 8. When T exists this is not
the case.
In the case therefore of electricity flowing out from a centre in an
infinite homogeneous, but not isotropic, medium, the equipotential
surfaces are ellipsoids, for each of which p is constant. The axes of
these ellipsoids are in the directions of the principal axes of con
ductivity, and these do not coincide with the principal axes of
resistance unless the system is symmetrical.
By a transformation of this equation we may take for the axes
of x, y, z the principal axes of conductivity. The coefficients of the
forms * and B will then be reduced to zero, and each coefficient
303.] SKEW SYSTEM. 349
of the form A will be the reciprocal of the corresponding1 coeffi
cient of the form r. The expression for p will be
^ + ^ + *! = ^!_. (22)
303.] The theory of the complete system of equations of resist
ance and of conductivity is that of linear functions of three vari
ables, and it is exemplified in the theory of Strains *, and in other
parts of physics. The most appropriate method of treating it is
that by which Hamilton and Tait treat a linear and vector function
of a vector. We shall not, however, expressly introduce Quaternion
notation.
The coefficients Tly T2, T3 may be regarded as the rectangular
components of a vector T, the absolute magnitude and direction
of which are fixed in the body, and independent of the direction of
the axes of reference. The same is true of ^, £2, #3, which are the
components of another vector t.
The vectors T and t do not in general coincide in direction.
Let us now take the axis of z so as to coincide with the vector
T, and transform the equations of resistance accordingly. They
will then have the form
X = XiU + SsV+Ssio-Tv,
Y = SsU+R^v + S^ + Tu, (23)
Z = Su + S
It appears from these equations that we may consider the elec
tromotive force as the resultant of two forces, one of them depending
only on the coefficients R and S, and the other depending on T alone.
The part depending on E and S is related to the current in the
same way that the perpendicular on the tangent plane of an
ellipsoid is related to the radius vector. The other part, depending
on T, is equal to the product of T into the resolved part of the
current perpendicular to the axis of T, and its direction is per
pendicular to T and to the current, being always in the direction in
which the resolved part of the current would lie if turned 90° in
the positive direction round T.
Considering the current and T as vectors, the part of the
electromotive force due to T is the vector part of the product,
T x current.
The coefficient T may be called the Rotatory coefficient. We
* See Thomson and Tail's Natural Philosophy. § 154.
350 RESISTANCE AND CONDUCTIVITY. [304.
have reason to believe that it does not exist in any known sub
stance. It should be found, if anywhere, in magnets, which have
a polarization in one direction, probably due to a rotational phe
nomenon in the substance.
304.] Let us next consider the general characteristic equation
ofF,
d dV dV dV d , dV dV dV
d dv
where the coefficients of conductivity p, q, r may have any positive
values, continuous or discontinuous, at any point of space, and V
vanishes at infinity.
Also, let #, 6, c be three functions of x, y, z satisfying the condition
da db dc
dV dV dV
and let a — r, -=— -f p~ -=- -f 09 — -
dx A dy a dz
__ dF dF dF
dx 2 dy dz
dV dV dV
(26)
Finally, let the triple-integral
be extended over spaces bounded as in the enunciation of Art. 97,
where the coefficients P, Q, R are the coefficients of resistance.
Then W will have a unique minimum value when a, b, c are such
that u} v, w are each everywhere zero, and the characteristic equation
(24) will therefore, as shewn in Art. 98, have one and only one
solution.
In this case W represents the mechanical equivalent of the heat
generated by the current in the system in unit of time, and we have
to prove that there is one way, and one only, of making this heat
a minimum, and that the distribution of currents (a be) in that case
is that which arises from the solution of the characteristic equation
of the potential V.
The quantity W may be written in terms of equations (25) and (26),
305.] EXTENSION OF THOMSON'S THEOREM. 351
? clTf d7*
\^Ty\^TZ
VdV dV dV
Since + +! = o, (29)
dk ay <£z
the third term of W vanishes within the limits.
The second term, being the rate of conversion of electrical energy
into heat, is also essentially positive. Its minimum value is zero,
and this is attained only when u, v, and w are everywhere zero.
The value of W is in this case reduced to the first term, and is
then a minimum and a unique minimum.
305.] As this proposition is of great importance in the theory of
electricity, it may be useful to present the following proof of the
most general case in a form free from analytical operations.
Let us consider the propagation of electricity through a conductor
of any form, homogeneous* or heterogeneous.
Then we know that
(1) If we draw a line along the path and in the direction of
the electric current, the line must pass from places of high potential
to places of low potential.
(2) If the potential at every point of the system be altered in
a given uniform ratio, the currents will be altered in the same ratio,
according to Ohm's Law.
(3) If a certain distribution of potential gives rise to a certain
distribution of currents, and a second distribution of potential gives
rise to a second distribution of currents, then a third distribution in
which the potential is the sum or difference of those in the first
and second will give rise to a third distribution of currents, such
that the total current passing through a given finite surface in the
third case is the sum or difference of the currents passing through
it in the first and second cases. For, by Ohm's Law, the additional
current due to an alteration of potentials is independent of the
original current due to the original distribution of potentials.
(4) If the potential is constant over the whole of a closed surface,
352 RESISTANCE AND CONDUCTIVITY. [3O5-
and if there are no electrodes or intrinsic electromotive forces
within it, then there will be no currents within the closed surface,
and the potential at any point within it will be equal to that at the
surface.
If there are currents within the closed surface they must either
be closed curves, or they must begin and end either within the
closed surface or at the surface itself.
But since the current must pass from places of high to places of
low potential, it cannot flow in a closed curve.
Since there are no electrodes within the surface the current
cannot begin or end within the closed surface, and since the
potential at all points of the surface is the same, there can be
no current along lines passing from one point of the surface to
another.
Hence there are no currents within the surface, and therefore
there can be no difference of potential, as such a difference would
produce currents, and therefore the potential within the closed
surface is everywhere the same as at the surface.
(5) If there is no electric current through any part of a closed
surface, and no electrodes or intrinsic electromotive forces within
the surface, there will be no currents within the surface, and the
potential will be uniform.
We have seen that the currents cannot form closed curves, or
begin or terminate within the surface, and since by the hypothesis
they do not pass through the surface, there can be no currents, and
therefore the potential is constant.
(6) If the potential is uniform over part of a closed surface, and
if there is no current through the remainder of the surface, the
potential within the surface will be uniform for the same reasons.
(7) If over part of the surface of a body the potential of every
point is known, and if over the rest of the surface of the body the
current passing through the surface at each point is known, then
only one distribution of potentials at points within the body can
exist.
For if there were two different values of the potential at any
point within the body, let these be FL in the first case and F2 in
the second case, and let us imagine a third case in which the
potential of every point of the body is the excess of potential in the
first case over that in the second. Then on that part of the surface
for which the potential is known the potential in the third case will
be zero, and on that part of the surface through which the currents
306.] RESISTANCE OF A WIRE OF VARIABLE SECTION. 353
are known the currents in the third case will be zero, so that by
(6) the potential everywhere within the surface will be zero, or
there is no excess of V^ over F9, or the reverse. Hence there is
only one possible distribution of potentials. This proposition is
true whether the solid be bounded by one closed surface or by
several.
On the Approximate Calculation of the Resistance of a Conductor
of a given Form.
306.] The conductor here considered has its surface divided into
three portions. Over one of these portions the potential is main
tained at a constant value. Over a second portion the potential has
a constant value different from the first. The whole of the remainder
of the surface is impervious to electricity. "We may suppose the
conditions of the first and second portions to be fulfilled by applying
to the conductor two electrodes of perfectly conducting material,
and that of the remainder of the surface by coating it with per
fectly non-conducting material.
Under these circumstances the current in every part of the
conductor is simply proportional to the difference between the
potentials of the electrodes. Calling this difference the electro
motive force, the total current from the one electrode to the other
is the product of the electromotive force by the conductivity of the
conductor as a whole, and the resistance of the conductor is the
reciprocal of the conductivity.
It is only when a conductor is approximately in the circumstances
above defined that it can be said to have a definite resistance, or
conductivity as a whole. A resistance coil, consisting of a thin
wire terminating in large masses of copper, approximately satisfies
these conditions, for the potential in the massive electrodes is nearly
constant, and any differences of potential in different points of the
same electrode may be neglected in comparison with the difference
of the potentials of the two electrodes.
A very useful method of calculating the resistance of such con
ductors has been given, so far as I know, for the first time, by
the Hon. J. W. Strutt, in a paper on the Theory of Resonance*.
It is founded on the following considerations.
If the specific resistance of any portion of the conductor be
changed, that of the remainder being unchanged, the resistance of
* Phil. Trans., 1871, p. 77. See Art. 102.
VOL. I. A a
354 RESISTANCE AND CONDUCTIVITY. [306.
the whole conductor will be increased if that of the portion is
increased, and diminished if that of the portion be diminished.
This principle may be regarded as self-evident, but it may easily
be shewn that the value of the expression for the resistance of a
system of conductors between two points selected as electrodes,
increases as the resistance of each member of the system in
creases.
It follows from this that if a surface of any form be described
in the substance of the conductor, and if we further suppose this
surface to be an infinitely thin sheet of a perfectly conducting
substance, the resistance of the conductor as a whole will be
diminished unless the surface is one of the equipotential surfaces
in the natural state of the conductor, in which case no effect will
be produced by making it a perfect conductor, as it is already in
electrical equilibrium.
If therefore we draw within the conductor a series of surfaces,
the first of which coincides with the first electrode, and the last
with the second, while the intermediate surfaces are bounded by
the non-conducting surface and do not intersect each other, and
if we suppose each of these surfaces to be an infinitely thin sheet
of perfectly conducting matter, we shall have obtained a system
the resistance of which is certainly not greater than that of the
original conductor, and is equal to it only when the surfaces we
have chosen are the natural equipotential surfaces.
To calculate the resistance of the artificial system is an operation
of much less difficulty than the original problem. For the resist
ance of the whole is the sum of the resistances of all the strata
contained between the consecutive surfaces, and the resistance of
each stratum can be found thus :
Let dS be an element of the surface of the stratum, v the thick
ness of the stratum perpendicular to the element, p the specific
resistance, E the difference of potential of the perfectly conducting
surfaces, and dC the current through dS, then
dC=E—dS, (1)
pv
and the whole current through the stratum is
L«4 (2)
pv
the integration being extended over the whole stratum bounded by
the non-conducting surface of the conductor.
306.] RESISTANCE OP A WIRE OF VARIABLE SECTION. 355
Hence the conductivity of the stratum is
' " ; '" • I =//>* ' • " '" <•>
and the resistance of the stratum is the reciprocal of this quantity.
If the stratum be that bounded by the two surfaces for which
the function F has the values F and F+ dF respectively, then
(5)
and the resistance of the stratum is
dF
1
P
To find the resistance of the whole artificial conductor, we have
only to integrate with respect to F, and we find
The resistance R of the conductor in its natural state is greater
than the value thus obtained, unless all the surfaces we have chosen
are the natural equipotential surfaces. Also, since the true value
of R is the absolute maximum of the values of R^ which can thus
be obtained, a small deviation of the chosen surfaces from the true
equipotential surfaces will produce an error of R which is com
paratively small.
This method of determining a lower limit of the value of the
resistance is evidently perfectly general, and may be applied to
conductors of any form, even when p, the specific resistance, varies
in any manner within the conductor.
The most familiar example is the ordinary method of determining
the resistance of a straight wire of variable section. In this case
the surfaces chosen are planes perpendicular to the axis of the
wire, the strata have parallel faces, and the resistance of a stratum
of section S and thickness ds is
' **!=*£• '.,..... (')
and that of the whole wire of length s is
where S' is the transverse section and is a function of s.
A a 2
356 RESISTANCE AND CONDUCTIVITY. [3°7-
This method in the case of wires whose section varies slowly
with the length gives a result very near the truth, but it is really
only a lower limit, for the true resistance is always greater than
this, except in the case where the section is perfectly uniform.
307.] To find the higher limit of the resistance, let us suppose
a surface drawn in the conductor to be rendered impermeable to
electricity. The effect of this must be to increase the resistance of
the conductor unless the surface is one of the natural surfaces of
flow. By means of two systems of surfaces we can form a set of
tubes which will completely regulate the flow, and the effect, if
there is any, of this system of impermeable surfaces must be to
increase the resistance above its natural value.
The resistance of each of the tubes may be calculated by the
method already given for a fine wire, and the resistance of the
whole conductor is the reciprocal of the sum of the reciprocals of
the resistances of all the tubes. The resistance thus found is greater
than the natural resistance, except when the tubes follow the
natural lines of flow.
In the case already considered, where the conductor is in the
form of an elongated solid of revolution, let us measure as along the
axis, and let the radius of the section at any point be 6. Let one
set of impermeable surfaces be the planes through the axis for each
of which $ is constant, and let the other set be surfaces of revolution
for which ^ _ ^2j (9)
where ty is a numerical quantity between 0 and 1 .
Let us consider a portion of one of the tubes bounded by the
surfaces <p and $ + ^$, \j/ and \l/ + d\js, x and x+dx.
The section of the tube taken perpendicular to the axis is
ydyd$ = \Wdty d$. (10)
If 6 be the angle which the tube makes with the axis
*. - - (11)
The true length of the element of the tube is dx sec 0, and its
true section is * i« <ty «ty cos 0,
so that its resistance is
T , A dx a
Let A=, and
307.] HIGHER AND LOWER LIMITS. 357
the integration being extended over the whole length, x, of the
conductor, then the resistance of the tube d\\r d$ is
.
and its conductivity is
To find the conductivity of the whole conductor, which is the
sum of the conductivities of the separate tubes, we must integrate
this expression between $ = 0 and $ = 277, and between \j/ = 0
and \fr = I . The result is
which may be less, but cannot be greater, than the true con
ductivity of the conductor.
When -y- is always a small quantity -j will also be small, and we
may expand the expression for the conductivity, thus
F=l(-4+t|-4>">
The first term of this expression, — , is that which we should
A.
have found by the former method as the superior limit of the con
ductivity. Hence the true conductivity is less than the first term
but greater than the whole series. The superior value of the
resistance is the reciprocal of this, or
If, besides supposing the flow to be guided by the surfaces <£ and
\ff, we had assumed that the flow through each tube is proportional
to d\j/ d$, we should have obtained as the value of the resistance
under this additional constraint
(17)
which is evidently greater than the former value, as it ought to be,
on account of the additional constraint. In Mr. Strutt's paper this
is the supposition made, and the superior limit of the resistance
there given has the value (17), which is a little greater than that
which we have obtained in (16).
358 KESISTANCE AND CONDUCTIVITY. [3°8.
308.] We shall now apply the same method to find the correction
which must be applied to the length of a cylindrical conductor of
radius a when it? extremity is placed in metallic contact with a
massive electrode, which we may suppose of a different metal.
For the lower limit of the resistance we shall suppose that an
infinitely thin disk of perfectly conducting matter is placed between
the end of the cylinder and the massive electrode, so as to bring
the end of the cylinder to one and the same potential throughout.
The potential within the cylinder will then be a function of its
length only, and if we suppose the surface of the electrode where
the cylinder meets it to be approximately plane, and all its dimen
sions to be large compared with the diameter of the cylinder, the
distribution of potential will be that due to a conductor in the form
of a disk placed in an infinite medium. See Arts. 152, 177.
If E is the difference of the potential of the disk from that of
the distant parts of the electrode, C the^current issuing from the
surface of the disk into the electrode, and /o' the specific resistance
of the electrode, p'Q. — ±aR (18)
Hence, if the length of the wire from a given point to the
electrode is L, and its specific resistance p, the resistance from that
point to any point of the electrode not near the junction is
TT a 4 $
and this may be written
JZ = JL(Z+£^), (19)
na* p 4 '
where the second term within brackets is a quantity which must
be added to the length of the cylinder or wire in calculating its
resistance, and this is certainly too small a correction.
To understand the nature of the outstanding error we may
observe, that whereas we have supposed the flow in the wire up
to the disk to be uniform throughout the section, the flow from
the disk to the electrode is not uniform, but is at any point in
versely proportional to the minimum chord through that point. In
the actual case the flow through the disk will not be uniform,
but it will not vary so much from point to point as in this supposed
case. The potential of the [disk in the actual case will not be
uniform, but will diminish from the middle to the edge.
309.] We shall next determine a quantity greater than the true
resistance by constraining the flow through the disk to be uniform
309.] CORRECTION FOR THE ENDS OF THE WIRE. 359
at every point. We may suppose electromotive forces introduced
for this purpose acting perpendicular to the surface of the disk.
The resistance within the wire will be the same as before, but
in the electrode the rate of generation of heat will be the surface-
integral of the product of the flow into the potential. The rate of
ri
flow at any point is — -2 , and the potential is the same as that of
an electrified surface whose surface-density is o-, where
.
p being the specific resistance.
We have therefore to determine the potential energy of the
electrification of the disk with the uniform surface-density o-.
The potential at the edge of a disk of uniform density a- is easily
found to be 4«cr. The work done in adding a strip of breadth
da at the circumference of the disk is 2 -naa-da . lav, and the
whole potential energy of the disk is the integral of this,
or P=— a* a*. (21)
o
In the case of electrical conduction the rate at which work is
done in the electrode whose resistance is Rf is
C*R=^P, (22)
P
whence, by (20) and (21),
and the correction to be added to the length of the cylinder is
P 8
7 Si*
this correction being greater than the true value. The true cor-
f
rection to be added to the length is therefore — an, where n is a
o P
number lying- between - and — , or between 0.785 and 0.849.
4 3?r
Mr. Strutt, by a second approximation, has reduced the superior
limit of n to 0.8282.
\\
CHAPTER IX.
CONDUCTION THROUGH HETEROGENEOUS MEDIA.
On the Conditions to be Fulfilled at the Surface of Separation
between Two Conducting Media.
310.] THERE are two conditions which the distribution of currents
must fulfil in general, the condition that the potential must be
continuous, and the condition of ' continuity' of the electric currents.
At the surface of separation between two media the first of these
conditions requires that the potentials at two points on opposite
sides of the surface, but infinitely near each other, shall be equal.
The potentials are here understood to be measured by an elec
trometer put in connexion with the given point by means of an
electrode of a given metal. If the potentials are measured by the
method described in Arts. 222,, 246, where the electrode terminates
in a cavity of the conductor filled with air, then the potentials at
contiguous points of different metals measured in this way will
differ by a quantity depending on the temperature and on the
nature of the two metals.
The other condition at the surface is that the current through
any element of the surface is the same when measured in either
medium.
Thus, if Fj and F2 are the potentials in the two media, then at
any point in the surface of separation
7i = r,, a)
and if nlf vlt w± and u2, v2) w2 are the components of currents in the
two media, and I, m, n the direction-cosines of the normal to the
surface of separation,
% I + #! m -f W-L n = u.2l-\-v2m + w%n. (2)
In the most general case the components n, v, w are linear
310.] SURFACE-CONDITIONS. 361
functions of the derivatives of Ft the forms of which are given in
the equations
u =
v = q3X+r2Y+plZ,> (3)
w = p2X+ q1Y+ r3Z,)
where X, Y, Z are the derivatives of V with respect to x, y, z
respectively.
Let us take the case of the surface which separates a medium
having these coefficients of conduction from an isotropic medium
having a coefficient of conduction equal to r.
Let X', Y'} Z' be the values of X, Y, Z in the isotropic medium,
then we have at the surface
r=r, (4)
or Xdx+Ydy + Zdz = X'dx+Y'dy + Z'dz, (5)
when Idx + mdy + ndz = 0. (6)
This condition gives
X'=X+47TO-^ 7'= 7+4770-02, Z' = Z+lll<Tn, (?)
where a- is the surface-density.
We have also in the isotropic medium
u'=rX', v'=rY', w'=rZ', (8)
and at the boundary the condition of flow is
u'l-\-tfm + w'n r= ul + vm + wn, (9)
or r(lX+mY+nZ+±ir<T}
= l(r1X+psY+ chZ)+m(^X+ r2Y+^Z) + »Q»2X+ q1Y+ r3Z), (10)
whence
477 or r = (l(ri — r) + mq3 + np^ X+ (Ip3 + m(r2 — r) + nq^Y
+ (lq2 + mj)1 + n(r2-r))Z. (11)
The quantity cr represents the sui-face-density of the charge
on the surface of separation. In crystallized and organized sub
stances it depends on the direction of the surface as well as on
the force perpendicular to it. In isotropic substances the coeffi
cients^; and q are zero, and the coefficients r are all equal, so that
4770-= (-^f- l)(lX+m7+nZ), (12)
where rx is the conductivity of the substance, r that of the external
medium, and I, m, n the direction-cosines of the normal drawn
towards the medium whose conductivity is r.
When both media are isotropic the conditions may be greatly
362 CONDUCTION IN HETEROGENEOUS MEDIA. [31 1.
simplified, for if k is the specific resistance per unit of volume, then
1 dV 1 dV l dV
u=—r-j-> #=____, w——, (13)
k dx k dy k dz
and if v is the normal drawn at any point of the surface of separation
from the first medium towards the second, the conduction of con
tinuity is 1 dV^ 1 dF2
&! dv ~ k2 dv
If 0j and 62 are the angles which the lines of flow in the first and
second media respectively make with the normal to the surface
of separation, then the tangents to these lines of flow are in the
same plane with the normal and on opposite sides of it, and
#! tan 61 = k2 tan 02. (15)
This may be called the law of refraction of lines of flow.
311.] As an example of the conditions which must be fulfilled
when electricity crosses the surface of separation of two media,
let us suppose the surface spherical and of radius #, the specific
resistance being ^ within and Jc% without the surface.
Let the potential, both within and without the surface, be ex
panded in solid harmonics, and let the part which depends on
the surface harmonic 8i be
t (2)
within and without the sphere respectively.
At the surface of separation where r = a we must have
F1=r2 and J-^'f^. (3)
^ dr k2 dr
From these conditions we get the equations
(*)
J. At
These equations are sufficient, when we know two of the four
quantities A19 A2, B^ B2, to deduce the other two.
Let us suppose A^ and Bl known, then we find the following
expressions for A2 and B»,
(5)
=
312.] SPHERICAL SHELL. 363
In this way we can find the conditions which each term of the
harmonic expansion of the potential must satisfy for any number of
strata bounded by concentric spherical surfaces.
312.] Let us suppose the radius of the first spherical surface
to be #15 and let there be a second spherical surface of radius a2
greater than alf beyond which the specific resistance is £3. If there
are no sources or sinks of electricity within these spheres there
will be no infinite values of T, and we shall have Bl = 0.
We then find for A3 and .Z?3, the coefficients for the outer medium,
*u.:-|
(6)
1)2 = [»&(»+
The value of the potential in the outer medium depends partly
on the external sources of electricity, which produce currents in
dependently of the existence of the sphere of heterogeneous matter
within, and partly on the disturbance caused by the introduction of
the heterogeneous sphere.
The first part must depend on solid harmonics of positive degrees
only, because it cannot have infinite values within the sphere. The
second part must depend on harmonics of negative degrees, because
it must vanish at an infinite distance from the centre of the sphere.
Hence the potential due to the external electromotive forces must
be expanded in a series of solid harmonics of positive degree. Let
A3 be the coefficient of one these, of the form
44*
Then we can find J.19 the corresponding coefficient for the inner
sphere by equation (6), and from this deduce A%, -Z?2> an^ ^3- Of
these _Z?3 represents the effect on the potential in the outer medium
due to the introduction of the heterogeneous spheres.
Let us now suppose £3 = k± , so that the case is that of a hollow
shell for which k = k2, separating an inner from an outer portion of
the same medium for which k=Jcl.
If we put
1
(2i^\^klk^i(i+i}(k2-klY(\-^
364: CONDUCTION IN HETEROGENEOUS MEDIA. [3*3-
then A1 = ^^2(2i+l)2CA3,
The difference between A3 the undisturbed coefficient, and A1 its
value in the hollow within the spherical shell, is
A3-A1 = (k2~k^ i(i+i)(i- (^)2* hl) CA3. (8)
Since this quantity is always positive whatever be the values
of k± and &2, it follows that, whether the spherical shell conducts
better or worse than the rest of the medium, the electrical action
within the shell is less than it would otherwise be. If the shell
is a better conductor than the rest of the medium it tends to
equalize the potential all round the inner sphere. If it is a worse
conductor, it tends to prevent the electrical currents from reaching
the inner sphere at all.
The case of a solid sphere may be deduced from this by making
#! = 0, or it may be worked out independently.
313.] The most important term in the harmonic expansion is
that in which i = 1, for which
c=
= 3&2 (2 h
(9)
The case of a solid sphere of resistance k2 may be deduced from
this by making a1 = 0. We then have
(10)
It is easy to shew from the general expressions that the value
of B9 in the case of a hollow sphere having a nucleus of resistance
£u surrounded by a shell of resistance k2, is the same as that of
a uniform solid sphere of the radius of the outer surface, and of
resistance K, where
314.] MEDIUM CONTAINING SMALL SPHERES. 365
314.] If there are n spheres of radius a^ and resistance Alt placed
in a medium whose resistance is k2) at such distances from each
other that their effects in disturbing the course of the current
may be taken as independent of each other, then if these spheres
are all contained within a sphere of radius #2, the potential at a
great distance from the centre of this sphere will be of the form
I
where the value of B is
B = AzA a* A. (13)
The ratio of the volume of the n small spheres to that of the
sphere which contains them is
na-,3
The value of the potential at a great distance from the sphere
may therefore be written
Now if the whole sphere of radius a.2 had been made of a material
of specific resistance K, we should have had
That the one expression should be equivalent to the other,
2*! +
=
This, therefore, is the specific resistance of a compound medium
consisting of a substance of specific resistance £2, in which are
disseminated small spheres of specific resistance < the ratio of the
volume of all the small spheres to that of the whole being p. In
order that the action of these spheres may not produce effects
depending on their interference, their radii must be small compared
with their distances, and therefore p must be a small fraction.
This result may be obtained in other ways, but that here given
involves only the repetition of the result already obtained for a
single sphere.
When the distance between the spheres is not great compared
% _ fc
with their radii, and when — ~ - j- is considerable, then other
2 #! -f- #2
terms enter into the result, which we shall not now consider.
In consequence of these terms certain systems of arrangement of
366 CONDUCTION IN HETEROGENEOUS MEDIA.
the spheres cause the resistance of the compound medium to be
different in different directions.
Application of the Principle of Images.
315.] Let us take as an example the case of two media separated
by a plane surface, and let us suppose that there is a source 8
of electricity at a distance a from the plane surface in the first
medium, the quantity of electricity flowing from the source in unit
of time being S.
If the first medium had been infinitely extended the current
at any point P would have been in the direction SP, and the
potential at P would have been - - where E = — - and ^ = SP.
In the actual case the conditions may be satisfied by taking
a point 7, the image of S in the second medium, such that 7$
is normal to the plane of separation and is bisected by it. Let r2
be the distance of any point from 7, then at the surface of separation
r-, = r0
25
_
~dv~ ~~dv'
Let the potential 7^ at any point in the first medium be that
due to a quantity of electricity E placed at S, together with an
imaginary quantity E2 at 7, and let the potential F2 at any point
of the second medium be that due to an imaginary quantity El at
8, then if
(3)
the superficial condition T± = F2 gives
and the condition
1 rlV. 1 dV-
(5)
"i
f\ 7_ 7 T
whence El =
The potential in the first medium is therefore the same as would
be produced in air by a charge E placed at S, and a charge E^
at 7 on the electrostatic theory, and the potential in the second
medium is the same as that which would be produced in air by
a charge El at S.
317.] STRATUM WITH PARALLEL SIDES. 367
The current at any point of the first medium is the same as would
k.—k
have been produced by the source S together with a source y — -^ S
placed at J if the first medium had been infinite, and the current
at any point of the second medium is the same as would have been
2 k S
produced by a source -r-, — — ,-r placed at S if the second medium had
(A + #2)
been infinite.
We have thus a complete theory of electrical images in the case
of two media separated by a plane boundary. Whatever be the
nature of the electromotive forces in the first medium, the potential
they produce in the first medium may be found by combining their
direct effect with the effect of their image.
If we suppose the second medium a perfect conductor, then
£2 = 0, and the image at / is equal and opposite to the course
at S. This is the case of electric images, as in Thomson's theory
in electrostatics.
If we suppose the second medium a perfect insulator, then
&2 = oc, and the image at / is equal to the source at S and of the
same sign. This is the case of images in hydrokinetics when the
fluid is bounded by a rigid plane surface.
316.] The method of inversion, which is of so much use in
electrostatics when the bounding surface is supposed to be that
of a perfect conductor, is not applicable to the more general case
of the surface separating two conductors of unequal electric resist
ance. The method of inversion in two dimensions is, however,
applicable, as well as the more general method of transformation in
two dimensions given in Art. 190 *.
Conduction through a Plate separating Two Media.
317.] Let us next consider the effect of a plate of thickness AB of
a medium whose resist
ance is £2, and separating ^
two media whose resist
ances are ^ and /£3, in ~£ J~~ J~
altering the potential due
to a source S in the first
medium.
The potential will be Fis- 23-
* See Kirchhoff, Pogg. Ann. Ixiv. 497, and Ixvii. 344 ; Quincke, Pogg. xcvii. 382 ;
and Smith, Proc. R. S. Edin., 1869-70, p. 79.
-
368 CONDUCTION IN HETEROGENEOUS MEDIA. [318.
equal to that due to a system of charges placed in air at certain
points along the normal to the plate through S.
Make
AI=SA, BI^SB, AJi=IiA9 BI^J^B, AJ2= I2A, &c. ;
then we have two series of points at distances from each other equal
to twice the thickness of the plate.
318.] The potential in the first medium at any point P is equal to
that at a point P* in the second
J^_
PI +
_
P'S + PI + p7/ + ~FI +
and that at a point P" in the third
where /, /', &c. represent the imaginary charges placed at the
points 7, &c., and the accents denote that the potential is to be
taken within the plate.
Then, by the last Article, for the surface through A we have,
Z. _ 7. o 7.
/=^_^1 E>=J^2K
^2 + ^1 *a + *i
For the surface through B we find
Similarly for the surface through A again,
j-f _ ^l~^2 jr T_ 2 &1 j
1 ~~ Je _U Jf l ' l ~ Tf JLb
K\ "r'*i K\ *<~ K2
and for the surface through B,
If If <>Jc
If we make _ &i—&2 -• / _
*l "f" *8 '
we find for the potential in the first medium,
' r
&c-
-. (15)
319.] STRATIFIED CONDUCTORS. 369
For the potential in the third medium we find
If the first medium is the same as the third, then k± = £3 and
p = //, and the potential on the other side of the plate will be
If the plate is a very much better conductor than the rest of the
medium, p is very nearly equal to 1 . If the plate is a nearly perfect
insulator, p is nearly equal to — 1, and if the plate differs little in
conducting power from the rest of the medium, p is a small quantity
positive or negative.
The theory of this case was first stated by Green in his ( Theory
of Magnetic Induction' (Essay, p. 65). His result, however, is
correct only when p is nearly equal to 1 *. The quantity g which
he uses is connected with p by the equations
2p _ #! — #a 3ff ._^i~4m
9 ~ 3-p ~~ £1+2/£2> P ~~ 2+ff~~ k-L + Jc^
p
If we put p = - , we shall have a solution of the problem of
1 + 27TK
the magnetic induction excited by a magnetic pole in an infinite
plate whose coefficient of magnetization is K.
On Stratified Conductors.
319.] Let a conductor be composed of alternate strata of thick
ness c and <f of two substances whose coefficients of conductivity
are different. Required the coefficients of resistance and conduc
tivity of the compound conductor.
Let the plane of the strata be normal to Z. Let every symbol
relating to the strata of the second kind be accented, and let
every symbol relating to the compound conductor be marked with
a bar thus, X. Then
X=X=X', (c+c'}u = cu + c'u,
Y=Y = Y', (c + c')v = cv + c'v';
(c -\-c')~Z = cZ+ c'Z', w = w — w'.
We must first determine «, u', #, ?/, Z and Z' in terms of
X, 7 and w from the equations of resistance, Art. 297, or those
* See Sir W. Thomson's ' Note on Induced Magnetism in a Plate,' Canib. and
Dub. Math. Journ., Nov. 1845, or Reprint, art. ix. § 156.
VOL. I. B b
370 CONDUCTION IN HETEROGENEOUS MEDIA. [320.
of conductivity, Art. 298. If we put D for the determinant of the
coefficients of resistance, we find
ur^D = R2X- Q3 Y+wq2D,
v r3 D = R! Y- P3 X + wft I),
Similar equations with the symbols accented give the values
of u, v' and /. Having found u, v and ~w in terms of X, F and ^
we may write down the equations of conductivity of the stratified
conductor. If we make h = — and //= — . we find
«
h + h' 1*=
=
__
_
'(Pl -
c + cf
320.] If neither of the two substances of which the strata are
formed has the rotatory property of Art. 303, the value of any
P or p will be equal to that of its corresponding Q or q. From
this it follows that in the stratified conductor also
or there is no rotatory property developed by stratification, unless
it exists in the materials.
321.] If we now suppose that there is no rotatory property, and
also that the axes of x, y and z are the principal axes, then the
p and q coefficients vanish, and
'7*2 + 6«V/ C + C'
7 )
C + C
322.] STRATIFIED CONDUCTORS, 371
If we begin with both substances isotropic, but of different
conductivities, then the result of stratification will be to make
the resistance greatest in the direction of a normal to the strata,
and the resistance in all directions in the plane of the strata will
be equal.
322.] Take an isotropic substance of conductivity r, cut it into
exceedingly thin slices of thickness a, and place them alternately
with slices of a substance whose conductivity is s, and thickness
k±a.
Let these slices be normal to x. Then cut this compound con
ductor into thicker slices, of thickness Z>, normal to y> and alternate
these with slices whose conductivity is s and thickness Jc.2 b.
Lastly, cut the new conductor into still thicker slices, of thick
ness c, normal to ^, and alternate them with slices whose con
ductivity is s and thickness £3 c.
The result of the three operations will be to cut the substance
whose conductivity is r into rectangular parallelepipeds whose
dimensions are «, b and c9 where b is exceedingly small compared
with c} and a is exceedingly small compared with b, and to embed
these parallelepipeds in the substance whose conductivity is s, so
that they are separated from each other l\a in the direction of x>
£2 b in that of y} and £3 c in that of z. The conductivities of the
conductor so formed in the directions of x, y and z are
_
3 £
The accuracy of this investigation depends upon the three
dimensions of .the parallelepipeds being of different orders of mag
nitude, so that we may neglect the conditions to be fulfilled at
their edges and angles. If we make kl} k2 and £3 each unity, then
3r+5s
If r — 0, that is, if the medium of which the parallelepipeds
are made is a perfect insulator, then
= f
B b 2
372 CONDUCTION IN HETEKOGENEOUS MEDIA. [323-
If r = oo, that is, if the parallelepipeds are perfect conductors,
ri = i*> rz = %*> r3 = 2s.
In every case, provided ^ = &2 = £3, it may be shewn that
rl9 r2 an(l rs are in ascending order of magnitude, so that the
greatest conductivity is in the direction of the longest dimensions
of the parallelepipeds, and the greatest resistance in the direction
of their shortest dimensions.
323.] In a rectangular parallelepiped of a conducting solid, let
there be a conducting channel made from one angle to the opposite,
the channel being a wire covered with insulating material, and
let the lateral dimensions of the channel be so small that the
conductivity of the solid is not affected except on account of the
current conveyed along the wire.
Let the dimensions of the parallelepiped in the directions of the
coordinate axes be <z, 6, <?, and let the conductivity of the channel,
extending from the origin to the point (adc), be abcK.
The electromotive force acting between the extremities of the
channel is aX+bY+cZ,
and if C' be the current along the channel
C' = Kalc(aX+bY+cZ).
The current across the face be of the parallelepiped is dcu, and
this is made up of that due to the conductivity of the solid and
of that due to the conductivity of the channel, or
bcu = bc
or a = (rl
In the same way we may find the values of v and w. The
coefficients of conductivity as altered by the effect of the channel
will be
In these expressions, the additions to the values of plt &c., due
to the effect of the channel, are equal to the additions to the values
of qlt &c. Hence the values of p-^ and q± cannot be rendered
unequal by the introduction of linear channels into every element
of volume of the solid, and therefore the rotatory property of
Art. 303, if it does not exist previously in a solid, cannot be
introduced by such means.
324-] COMPOSITE CONDUCTOR. 373
324.] To construct a framework of linear conductors which shall
have any given coefficients of conductivity forming a symmetrical
system.
Let the space be divided into equal small
cubes, of which let the figure represent one.
Let the coordinates of the points 0, Ly M, N,
ntials
be as
x
A
follows :
y z Potential.
\L/'
i/°\
N' M
0
0
0
0
0
Fig. 24.
L
0
1
1
V + Y+Z,
M
1
0
1
Q+Z+X,
N
1
1
0
0+X+Y.
Let these four points be connected by six conductors,
OL, OH, ON, 3IN, NL, LM,
of which the conductivities are respectively
A, JS, C, P, Q9 R.
The electromotive forces along these conductors will be
Y+Z, Z+X, X+Y, Y-Z, Z-X, X-Y,
and the currents
A(Y+Z), 3 (Z+X), C(X+Y), P(Y-Z), Q(Z-X), R(X-Y).
Of these currents, those which convey electricity in the positive
direction of x are those along LM, LN, OH and ON, and the
quantity conveyed is
„ = (B
Similarly
v = (C-R}X
w = (3-Q)X +(A-P)Y
whence we find by comparison with the equations of conduction,
Art. 298,
4: A = r2 + rs-r1 + 2dp1, 4P =
±£= rB + r1-r2 + 2p2, 4 Q =
4 G =
CHAPTER X.
CONDUCTION IN DIELECTRICS.
325.] WE have seen that when electromotive force acts on a
dielectric medium it produces in it a state which we have called
electric polarization, and which we have described as consisting*
of electric displacement within the medium in a direction which,
in isotropic media,, coincides with that of the electromotive force,
combined with a superficial charge on every element of volume
into which we may suppose the dielectric divided, which is negative
on the side towards which the force acts, and positive on the side
from which it acts.
When electromotive force acts on a conducting medium it also
produces what is called an electric current.
Now dielectric media, with very few, if any, exceptions, are
also more or less imperfect conductors, and many media which are
not good insulators exhibit phenomena of dielectric induction.
Hence we are led to study the state of a medium in which induction
and conduction are going on at the same time.
-For simplicity we shall suppose the medium isotropic at every
point, but not necessarily homogeneous at different points. In this
case, the equation of Poisson becomes, by Art. 83,
d (vdV^ d f^dV^ d ,vdY^
«?^*^<*^*«C^)*-*»^ a)
where K is the ' specific inductive capacity.'
The * equation of continuity' of electric currents becomes
i<iiS\:L- (l*I\ d(idr. dp_
dx V ifoJ + dy V dy> + dz V fa) ~ Tt (}
where r is the specific resistance referred to unit of volume.
When K or r is discontinuous, these equations must be trans
formed into those appropriate to surfaces of discontinuity.
326.] THEORY OF A CONDENSER. 375
In a strictly homogeneous medium r and K are both constant, so
that we find
d*V d*V d*V P dp ,ox
-7-T+-7^+-r¥=-47r-^=r:£, (3)
dx2 dj/2 dz2 K at
-*Zt
whence p = Ce Kr ; (4)
Kr -L
or, if we put T= —, p —Ce ?. (5)
This result shews that under the action of any external electric
forces on a homogeneous medium, the interior of which is originally
charged in any manner with electricity, the internal charges will
die away at a rate which does not depend on the external forces,
so that at length there will be no charge of electricity within
the medium, after which no external forces can either produce or
maintain a charge in any internal portion of the medium, pro
vided the relation between electromotive force, electric polarization
and conduction remains the same. When disruptive discharge
occurs these relations cease to be true, and internal charge may
be produced.
On Conduction through a Condenser.
326.] Let C be the capacity of a condenser, R its resistance, and
E the electromotive force which acts on it, that is, the difference of
potentials of the surfaces of the metallic electrodes.
Then the quantity of electricity on the side from which the
electromotive force acts will be CE, and the current through the
substance of the condenser in the direction of the electromotive
E
force will be -^>
H
If the electrification is supposed to be produced by an electro
motive force E acting in a circuit of which the condenser forms
part, and if -^ represents the current in that circuit, then
9-!+«"-
Let a battery of electromotive force EQ and resistance i\ be
introduced into this circuit, then
,
Hence, at any time tlt
(8)
376 CONDUCTION IN DIELECTRICS. [327.
Next, let the circuit r± be broken for a time t2,
_^_
E(=E^=E^e TZ where T2 = CR. (9)
Finally, let the surfaces of the condenser be connected by means
of a wire whose resistance is r3 for a time tz ,
E(=E3) = E2e-% where T, = |^A. (10)
If Qs is the total discharge through this wire in the time £3,
In this way we may find the discharge through a wire which
is made to connect the surfaces of a condenser after being charged
for a time tlt and then insulated for a time t2. If the time of
charging is sufficient, as it generally is, to develope the whole
charge, and if the time of discharge is sufficient for a complete
discharge, the discharge is
-*-
3.27.] In a condenser of this kind, first charged in any way, next
discharged through a wire of small resistance, and then insulated,
no new electrification will appear. In most actual condensers,
however, we find that after discharge and insulation a new charge
is gradually developed, of the same kind as the original charge,
but inferior in intensity. This is called the residual charge. To
account for it we must admit that the constitution of the dielectric
medium is different from that which we have just described. We
shall find, however, that a medium formed of a conglomeration of
small pieces of different simple media would possess this property.
Theory of a Composite Dielectric.
328.] We shall suppose, for the sake of simplicity, that the
dielectric consists of a number of plane strata of different materials
and of area unity, and that the electric forces act in the direction
of the normal to the strata.
Let al9 #2> &c. be the thicknesses of the different strata.
Xlt X2, &c. the resultant electrical force within each stratum.
fli,p2> &c' ^ne current due to conduction through each stratum.
fi>fz> &c- ^ne electric displacement.
ult ^2, &c. the total current, due partly to conduction and partly
to variation of displacement.
328.] STRATIFIED DIELECTRIC. 377
r1} r.2, &c. the specific resistance referred to unit of volume.
K1} K2, &c. the specific inductive capacity.
£15 £2, &c. the reciprocal of the specific inductive capacity.
E the electromotive force due to a voltaic battery, placed in
the part of the circuit leading from the last stratum towards the
first, which we shall suppose good conductors.
Q the total quantity of electricity which has passed through this
part of the circuit up to the time t.
EQ the resistance of the battery with its connecting wires.
o-^ the surface-density of electricity on the surface which separates
the first and second strata.
Then in the first stratum we have, by Ohm's Law,
By the theory of electrical displacement,
*,= 4»V1. (2)
By the definition of the total current,
_
with similar equations for the other strata, in each of which the
quantities have the suffix belonging to that stratum.
To determine the surface-density on any stratum, we have an
equation of the form ^ — f — f9 /4)
and to determine its variation we have
f/0-19 ,r\
-£=*-*•
By differentiating (4) with respect to z5, and equating the result
to (5), we obtain
o
- = «,sa7> (6)
or, by taking account of (3),
u^ = u2 = &c. = u. (7)
That is, the total current u is the same in all the strata, and is
equal to the current through the wire and battery.
We have also, in virtue of equations (1) and (2),
1 . 1 dX,
u = ^^+j^^'
from which we may find Xl by the inverse operation on u,
di
378 CONDUCTION IN DIELECTRICS. [329.
The total electromotive force E is
E = a1X1 + a2X2 + &Lc.) (10)
an equation between E, the external electromotive force, and u, the
external current.
If the ratio of r to k is the same in all the strata, the equation
reduces itself to
j (12)
which is the case we have already examined, and in which, as we
found, no phenomenon of residual charge can take place.
If there are n substances having different ratios of r to k, the
general equation (11), when cleared of inverse operations, will be
a linear differential equation, of the nth order with respect to E
and of the (n— l)th order with respect to u, t being the independent
variable.
From the form of the equation it is evident that the order of
the different strata is indifferent, so that if there are several strata
of the same substance we may suppose them united into one
without altering the phenomena.
329.] Let us now suppose that at first fi,f2) &c. are all zero,
and that an electromotive force E is suddenly made to act, and let
us find its instantaneous effect.
Integrating (8) with respect to t, we find
q = udt = TXi dt + -j- X1 + const. (13)
Now, since Xx is always in this case finite, / X± dt, must be in
sensible when t is insensible, and therefore, since X± is originally
zero, the instantaneous effect will be
Xl = 47i^Q. (14)
Hence, by equation (10),
E= 47r(£1tf1 + /£2tf2 + &c.)§, (15)
and if C be the electric capacity of the system as measured in this
instantaneous way,
__ Q __ _ 1 (16)
E 4w(£101 + £2«a + &c.)
329.] ELECTRIC ' ABSORPTION/ 379
This is the same result that we should have obtained if we had
neglected the conductivity of the strata.
Let us next suppose that the electromotive force E is continued
uniform for an indefinitely long time, or till a uniform current of
conduction equal top is established through the system.
We have then X1 = i\p, and therefore
E = (y1fl1 + /2«2 + &c.)Jp. (17)
If R be the total resistance of the system,
" P ~
In this state we have by (2),
so that ^(L.- __),, ••'... (19)
If we now suddenly connect the extreme strata by means of a
conductor of small resistance, E will be suddenly changed from its
original value E0 to zero, and a quantity Q of electricity will pass
through the conductor.
To determine Q we observe that if Xf be the new value of Xl ,
then by (13), j-/= X1 + 4 77 ^ Q. (20)
Hence, by (10), putting E = 0,
0 = ^ Xl + &c. + 4 77 (a1 k\ + az k.2 + &c.) Q, (21)
or 0 = ^0 + -^ Q. (22)
Hence Q = — C?^ where (7 is the capacity, as given by equation
(16). The instantaneous discharge is therefore equal to the in
stantaneous charge.
Let us next suppose the connexion broken immediately after this
discharge. We shall then have u = 0, so that by equation (8),
Xi = X'e i , (23)
where X' is the initial value after the discharge.
Hence, at any time t,
The value of E at any time is therefore
380 CONDUCTION IN DIELECTKICS. [33°-
and the instantaneous discharge after any time t is EC. This is
called the residual discharge.
If the ratio of r to k is the same for all the strata, the value of E
will be reduced to zero. If, however, this ratio is not the same, let
the terms be arranged according to the values of this ratio in
descending order of magnitude.
The sum of all the coefficients is evidently zero, so that when
t = 0, E = 0. The coefficients are also in descending order of
magnitude, and so are the exponential terms when t is positive.
Hence, when t is positive, E will be positive, so that the residual
discharge is always of the same sign as the primary discharge.
When t is indefinitely great all the terms disappear unless any
of the strata are perfect insulators, in which case r± is infinite for
that stratum/ and R is infinite for the whole system, and the final
value of E is not zero but
E = ^0(l-47ra1^1(7). (25)
Hence, when some, but not all, of the strata are perfect insulators,
a residual discharge may be permanently preserved in the system.
330.] "We shall next determine the total discharge through a wire
of resistance RQ kept permanently in connexion with the extreme
strata of the system, supposing the system first charged by means
of a long-continued application of the electromotive force E.
At any instant we have
E= a1rlpl + azr2p2 + &c. + JR0w = 0, (26)
and also, by (3), u=^L + . (27)
Hence (R + R0) * = «i *i ^ + V2 %& +&c. (28)
Integrating with respect to t in order to find Q, we get
(R + JR0)Q = «! r, (// -/J + a2 r2 (/2'-/2) + &c., (29)
where f^ is the initial, and/1/ the final value ofj^.
In this case // = 0, and /, = E, ( - «?) •
Hence (R + BJ Q = + + &<s.'-3>CX, (30)
where the summation is extended to all quantities of this form
belonging to every pair of strata.
331.] RESIDUAL DISCHARGE. 381
It appears from this that Q is always negative, that is to say, in
the opposite direction to that of the current employed in charging1
the system.
This investigation shews that a dielectric composed of strata of
different kinds may exhibit the phenomena known as electric
absorption and residual discharge, although none of the substances
of which it is made exhibit these phenomena when alone. An
investigation of the cases in which the materials are arranged
otherwise than in strata would lead to similar results, though
the calculations would be more complicated, so that we may
conclude that the phenomena of electric absorption may be ex
pected in the case of substances composed of parts of different
kinds, even though these individual parts should be microscopically
small.
It by no means follows that every substance which exhibits this
phenomenon is so composed, for it may indicate a new kind of
electric polarization of which a homogeneous substance may be
capable, and this in some cases may perhaps resemble electro
chemical polarization much more than dielectric polarization.
The object of the investigation is merely to point out the true
mathematical character of the so-called electric absorption, and to
shew how fundamentally it differs from the phenomena of heat
which seem at first sight analogous.
331.] If we take a thick plate of any substance and heat it
on one side, so as to produce a flow of heat through it, and if
we then suddenly cool the heated side to the same temperature
as the other, and leave the plate to itself, the heated side of the
plate will again become hotter than the other by conduction from
within.
Now an electrical phenomenon exactly analogous to this can
be produced, and actually occurs in telegraph cables, but its mathe
matical laws, though exactly agreeing with those of heat, differ
entirely from those of the stratified condenser.
In the case of heat there is true absorption of the heat into
the substance with the result of making it hot. To produce a truly
analogous phenomenon in electricity is impossible, but we may
imitate it in the following way in the form of a lecture-room
experiment.
Let Alt A29 &c. be the inner conducting surfaces of a series of
condensers, of which HQ, £lt H.2, &c. are the outer surfaces.
Let A19 A2, &c. be connected in series by connexions of resist-
382
CONDUCTION IN DIELECTRICS.
[33r-
ance R, and let a current be passed along this series from left to
right.
Let us first suppose the plates BQ, Rlf £2, each insulated and
free from charge. Then the total quantity of electricity on each of
the plates B must remain zero, and since the electricity on the
plates A is in each case equal and opposite to that of the opposed
A
Fig. 25.
surface they will not be electrified, and no alteration of the current
will be observed.
But let the plates B be all connected together, or let each be
connected with the earth. Then, since the potential of Al is
positive, while that of the plates B is zero, Al will be positively
electrified and B1 negatively.
If PU P2) &c. are the potentials of the plates Alt A2, &c., and C
the capacity of each, and if we suppose that a quantity of electricity
equal to Q0 passes through the wire on the left, Ql through the
connexion R^ and so on, then the quantity which exists on the
plate Al is Q0 — Q1 , and we have
Similarly Qi — Q:
and so on.
But by Ohm's Law we have
If we suppose the values of C the same for each plate, and those
of R the same for each wire, we shall have a series of equations of
the form
332.] THEORY OP ELECTRIC CABLES. 383
If there are n quantities of electricity to be determined, and if
either the total electromotive force, or some other equivalent con
ditions be given, the differential equation for determining any one
of them will be linear and of the nth order.
By an apparatus arranged in this way, Mr. Varley succeeded in
imitating the electrical action of a cable 12,000 miles long.
When an electromotive force is made to act along the wire on
the left hand, the electricity which flows into the system is at first
principally occupied in charging the different condensers beginning
with AI} and only a very small fraction of the current appears
at the right hand till a considerable time has elapsed. If galvano
meters be placed in circuit at S19 jR.2, &c. they will be affected
by the current one after another, the interval between the times of
equal indications being greater as we proceed to the right.
332.] In the case of a telegraph cable the conducting wire is
separated from conductors outside by a cylindrical sheath of gutta-
percha, or other insulating material. Each portion of the cable
thus becomes a condenser, the outer surface of which is always at
potential zero. Hence, in a given portion of the cable, the quantity
of free electricity at the surface of the conducting wire is equal
to the product of the potential into the capacity of the portion of
the cable considered as a condenser.
If a1} a2 are the outer and inner radii of the insulating sheath,
and if K is its specific dielectric capacity, the capacity of unit of
length of the cable is, by Art. 126,
* = -£-. CD
Let v be the potential at any point of the wire, which we may
consider as the same at every part of the same section.
Let Q be the total quantity of electricity which has passed
through that section since the beginning of the current. Then the
quantity which at the time t exists between sections at x and at
r, is n $n
and this is, by what we have said, equal to cvbx.
384 CONDUCTION IN DIELECTRICS. [333-
Hence cv=-^. (2)
clx
Again, the electromotive force at any section is — --, and by
Ohm's Law, ^ ^Q
__ = £-J|, (3)
dx dt
where k is the resistance of unit of length of the conductor, and
-~^ is the strength of the current. Eliminating Q between (2) and
dt
(3), we find , dv d2v ,.^
C/C ~j~ = "7— n ' (*)
dt d&
This is the partial differential equation which must be solved
in order to obtain the potential at any instant at any point of the
cable. It is identical with that which Fourier gives to determine
the temperature at any point of a stratum through which heat
is flowing in a direction normal to the stratum. In the case of
heat c represents the capacity of unit of volume, or what Fourier
calls CD, and k represents the reciprocal of the conductivity.
If the sheath is not a perfect insulator, and if k± is the resist
ance of unit of length of the sheath to conduction through it in a
radial direction, then if p± is the specific resistance of the insulating
material, r
*i=2Pllogef. (5)
'2
The equation (2) will no longer be true, for the electricity is
expended not only in charging the wire to the extent represented
v
by cv, but in escaping at a rate represented by -y- . Hence the rate
of expenditure of electricity will be
dv_ 1_ ,
° dt +
whence, by comparison with (3), we get
,dv
f.
-^
and this is the equation of conduction of heat in a rod or ring
as given by Fourier*.
333.] If we had supposed that a body when raised to a high
potential becomes electrified throughout its substance as if elec
tricity were compressed into it, we should have arrived at equa
tions of this very form. It is remarkable that Ohm himself,
* Theorie de la Chaleur, art. 105-
334-]
HYDROSTATICAL ILLUSTRATION.
385
misled by the analogy between electricity and heat, entertained
an opinion of this kind, and was thus, by means of an erroneous
opinion, led to employ the equations of Fourier to express the
true laws of conduction of electricity through a long wire, long
before the real reason of the appropriateness of these equations had
been suspected.
Mechanical Illustration of the Properties of a Dielectric.
334.] Five tubes of equal sectional area A, B, C, D and P are
arranged in circuit as in the figure.
A, B, C and D are vertical and equal,
and P is horizontal.
The lower halves of A} B, C, D
are filled with mercury, their upper
halves and the horizontal tube P are
filled with water.
A tube with a stopcock Q con
nects the lower part of A and B
with that of C and D, and a piston
P is made to slide in the horizontal
tube.
Let us begin by supposing that
the level of the mercury in the four
tubes is the same, and that it is
indicated by AQ, BQ, (70, DQ) that
the piston is at P0, and that the
stopcock Q is shut.
Now let the piston be moved from P0 to Pl} a distance a. Then,
since the sections of all the tubes are equal, the level of the mercury
in A and C will rise a distance a, or to A± and Clt and the mercury
in B and D will sink an equal distance a, or to B^ and D1.
The difference of pressure on the two sides of the piston will
be represented by 4#.
This arrangement may serve to represent the state of a dielectric
acted on by an electromotive force 4 a.
The excess of water in the tube D may be taken to represent
a positive charge of electricity on one side of the dielectric, and the
excess of mercury in the tube A may represent the negative charge
on the other side. The excess of pressure in the tube P on the
side of the piston next D will then represent the excess of potential
on the positive side of the dielectric.
VOL. i. c c
s — i^\
f p p p X
/ !° f »' \
-A -
(:
.— •- -^
^
- c -
-i
*
-A -
-B -
2
8
~A0-
~B0~
•v
-°0-
-Ca-
'*.-
-a -
-D -
/
i
t
Q
Fig. 26.
386 CONDUCTION IN DIELECTEICS. [334-
If the piston is free to move it will move back to P0 and be
in equilibrium there. This represents the complete discharge of
the dielectric.
During the discharge there is a reversed motion of the liquids
throughout the whole tube, and this represents that change of
electric displacement which we have supposed to take place in a
dielectric.
I have supposed every part of the system of tubes filled with
incompressible liquids, in order to represent the property of all
electric displacement that there is no real accumulation of elec
tricity at any place.
Let us now consider the effect of opening the stopcock Q while
the piston P is at Pl .
The level of AL and Dl will remain unchanged, but that of £ and
C will become the same, and will coincide with BQ and C0 .
The opening of the stopcock Q corresponds to the existence of
a part of the dielectric which has a slight conducting power, but
which does not extend through the whole dielectric so as to form
an open channel.
The charges on the opposite sides of the dielectric remain in
sulated, but their difference of potential diminishes.
In fact, the difference of pressure on the two sides of the piston
sinks from 4# to 2 a during the passage of the fluid through Q.
If we now shut the stopcock Q and allow the piston P to move
freely, it will come to equilibrium at a point P2, and the discharge
will be apparently only half of the charge.
The level of the mercury in A and B will be ^a above its
original level, and the level in the tubes C and D will be \a
below its original level. This is indicated by the levels A29 -Z?2,
c2, A-
If the piston is now fixed and the stopcock opened, mercury will
flow from B to C till the level in the two tubes is again at BQ and
C0. There will then be a difference of pressure = a on the two
sides of the piston P. If the stopcock is then closed and the piston
P left free to move, it will again come to equilibrium at a point P3,
half way between P2 and P0. This corresponds to the residual
charge which is observed when a charged dielectric is first dis
charged and then left to itself. It gradually recovers part of its
charge, and if this is again discharged a third charge is formed, the
successive charges diminishing in quantity. In the case of the
illustrative experiment each charge is half of the preceding, and the
334-] HYDROSTATICAL ILLUSTRATION. 387
discharges, which are J, ^, &c. of the original charge, form a series
whose sum is equal to the original charge.
If, instead of opening and closing the stopcock, we had allowed it
to remain nearly, but not quite, closed during the whole experiment,
we should have had a case resembling that of the electrification of a
dielectric which is a perfect insulator and yet exhibits the phe
nomenon called * electric absorption/
To represent the case in which there is true conduction through
the dielectric we must either make the piston leaky, or we must
establish a communication between the top of the tube A and the
top of the tube D.
In this way we may construct a mechanical illustration of the
properties of a dielectric of any kind, in which the two electricities
are represented by two real fluids, and the electric potential is
represented by fluid pressure. Charge and discharge are repre
sented by the motion of the piston P, and electromotive force by
the resultant force on the piston.
c c 2
CHAPTER XL
THE MEASUREMENT OF ELECTRIC RESISTANCE.
335.] IN the present state of electrical science, the determination
of the electric resistance of a conductor may be considered as the
cardinal operation in electricity, in the same sense that the deter
mination of weight is the cardinal operation in chemistry.
The reason of this is that the determination in absolute measure
of other electrical magnitudes, such as quantities of electricity,
electromotive forces, currents, &c., requires in each case a com
plicated series of operations, involving- generally observations of
time, measurements of distances, and determinations of moments
of inertia, and these operations, or at least some of them, must
be repeated for every new determination, because it is impossible
to preserve a unit of electricity, or of electromotive force, or of
current, in an unchangeable state, so as to be available for direct
comparison.
But when the electric resistance of a properly shaped conductor
of a properly chosen material has been once determined, it is found
that it always remains the same for the same temperature, so that
the conductor may be used as a standard of resistance, with which
that of other conductors can be compared, and the comparison of
two resistances is an operation which admits of extreme accuracy.
When the unit of electrical resistance has been fixed on, material
copies of this unit, in the form of ' Resistance Coils,' are prepared
for the use of electricians, so that in every part of the world
electrical resistances may be expressed in terms of the same unit.
These unit resistance coils are at present the only examples of
material electric standards which can be preserved, copied, and used
for the purpose of measurement. Measures of electrical capacity,
which are also of great importance, are still defective, on account
of the disturbing influence of electric absorption.
336.] The unit of resistance may be an entirely arbitrary one,
as in the case of Jacobins Etalon, which was a certain copper
wire of 22.4932 grammes weight, 7.61975 metres length, and 0.667
339-] STANDARDS OF RESISTANCE. 389
millimetres diameter. Copies of this have been made by Leyser of
Leipsig, and are to be found in different places.
According- to another method the unit may be defined as the
resistance of a portion of a definite substance of definite dimensions.
Thus, Siemens' unit is defined as the resistance of a column of
mercury of one metre long-, and one square millimetre section, at
the temperature 0°C.
337.] Finally, the unit may be defined with reference to the
electrostatic or the electromagnetic system of units. In practice
the electromagnetic system is used in all telegraphic operations,
and therefore the only systematic units actually in use are those
of this system.
In the electromagnetic system, as we shall shew at the proper
place, a resistance is a quantity homogeneous with a velocity, and
may therefore be expressed as a velocity. See Art. 628.
338.] The first actual measurements on this system were made
by Weber, who employed as his unit one millimetre per second.
Sir W. Thomson afterwards used one foot per second as a unit,
but a large number of electricians have now agreed to use the
unit of the British Association, which professes to represent a
resistance which, expressed as a velocity, is ten millions of metres
per second. The magnitude of this unit is more convenient than
that of Weber's unit, which is too small. It is sometimes referred
to as the B.A. unit, but in order to connect it with the name of
the discoverer of the laws of resistance, it is called the Ohm.
339.] To recollect its value in absolute measure it is useful
to know that ten millions of metres is professedly the distance
from the pole to the equator, measured along the meridian of Paris.
A body, therefore, which in one second travels along a meridian
from the pole to the equator would have a velocity which, on the
electromagnetic system, is professedly represented by an Ohm.
I say professedly, because, if more accurate researches should
prove that the Ohm, as constructed from the British Association's
material standards, is not really represented by this velocity, elec
tricians would not alter their standards, but would apply a cor
rection. In the same way the metre is professedly one ten-millionth
of a certain quadrantal arc, but though this is found not to be
exactly true, the length of the metre has not been altered, but the
dimensions of the earth are expressed by a less simple number.
According to the system of the British Association, the absolute
value of the unit is originally chosen so as to represent as nearly
390
MEASUREMENT OF RESISTANCE.
[340.
as possible a quantity derived from the electromagnetic absolute
system.
340.] When a material unit representing this abstract quantity
has been made, other standards are constructed by copying this unit,
a process capable of extreme accuracy — of much greater accuracy
than, for instance, the copying of foot-rules from a standard foot.
These copies, made of the most permanent materials, are dis
tributed over all parts of the world, so that it is not likely that
any difficulty will be found in obtaining copies of them if the
original standards should be lost.
But such units as that of Siemens can without very great
labour be reconstructed with considerable accuracy, so that as the
relation of the Ohm to Siemens unit is known, the Ohm can be
reproduced even without having a standard to copy, though the
labour is much greater and the accuracy much less than by the
method of copying.
Finally, the Ohm may be reproduced
by the electromagnetic method by which
it was originally determined. This method,
which is considerably more laborious than
the determination of a foot from the seconds
pendulum, is probably inferior in accuracy
to that last mentioned. On the other hand,
the determination of the electromagnetic
unit in terms of the Ohm with an amount
of accuracy corresponding to the progress
of electrical science, is a most important
physical research and well worthy of being
repeated.
The actual resistance coils constructed
to represent the Ohm were made of an
alloy of two parts of silver and one of pla
tinum in the form of wires from .5 milli
metres to .8 millimetres diameter, and from
one to two metres in length. These wires
were soldered to stout copper electrodes.
The wire itself was covered with two layers
Fig. 27.
of silk, imbedded in solid paraffin, and enclosed in a thin brass
case, so that it can be easily brought to a temperature at which
its resistance is accurately one Ohm. This temperature is marked
on the insulating support of the coil. (See Fig. 27.)
341-] RESISTANCE COILS. 391
0)i the Forms of Resistance Coils.
341.] A Resistance Coil is a conductor capable of being1 easily
placed in the voltaic circuit, so as to introduce into the circuit
a known resistance.
The electrodes or ends of the coil must be such that no appre
ciable error may arise from the mode of making the connexions.
For resistances of considerable magnitude it is sufficient that the
electrodes should be made of stout copper wire or rod well amal
gamated with mercury at the ends, and that the ends should be
made to press on flat amalgamated copper surfaces placed in mercury
cups.
For very great resistances it is sufficient that the electrodes
should be thick pieces of brass, and that the connexions should
be made by inserting a wedge of brass or copper into the interval
between them. This method is found very convenient.
The resistance coil itself consists of a wire well covered with
silk, the ends of which are soldered permanently to the elec
trodes.
The coil must be so arranged that its temperature may be easily
observed. For this purpose the wire is coiled on a tube and
covered with another tube, so that it may be placed in a vessel
of water, and that the water may have access to the inside and the
outside of the coil.
To avoid the electromagnetic effects of the current in the coil
the wire is first doubled back on itself and then coiled on the tube,
so that at every part of the coil there are equal and opposite
currents in the adjacent parts of the wire.
When it is desired to keep two coils at the same temperature the
wires are sometimes placed side by side and coiled up together.
This method is especially useful when it is more important to
secure equality of resistance than to know the absolute value of
the resistance, as in the case of the equal arms of Wheatstone's
Bridge, (Art. 347).
AVhen measurements of resistance were first attempted, a resist
ance coil, consisting of an uncovered wire coiled in a spiral groove
round a cylinder of insulating material, was much used. It was
called a Rheostat. The accuracy with which it was found possible
to compare resistances was soon found to be inconsistent with the
use of any instrument in which the contacts are not more perfect
than can be obtained in the rheostat. The rheostat, however, is
392
MEASUREMENT OF RESISTANCE.
[342.
still used for adjusting the resistance where accurate measurement is
not required.
Resistance coils are generally made of those metals whose resist
ance is greatest and which vary least with temperature. German
silver fulfils these conditions very well, but some specimens are
found to change their properties during the lapse of years. Hence
for standard coils, several pure metals, and also an alloy of platinum
and silver, have been employed, and the relative resistance of these
during several years has been found constant up to the limits of
modern accuracy.
342.] For very great resistances, such as several millions of
Ohms, the wire must be either very long or very thin, and the
construction of the coil is expensive and difficult. Hence tellurium
and selenium have been proposed as materials for constructing
standards of great resistance. A very ingenious and easy method
of construction has been lately proposed by Phillips *. On a piece
of ebonite or ground glass a fine pencil-line is drawn. The ends
of this filament of plumbago are connected to metallic electrodes,
and the whole is then covered with insulating varnish. If it
should be found that the resistance of such a pencil-line remains
constant, this will be the best method of obtaining a resistance of
several millions of Ohms.
343.] There are various arrangements by which resistance coils
may be easily introduced into a circuit.
For instance, a series of coils of which the resistances are 1,2,
4, 8, 16, &c., arranged according to the powers of 2, may be placed
in a box in series.
G4 32
/&•
Fig. 28.
The electrodes consist of stout brass plates, so arranged on the
outside of the box that by inserting a brass plug or wedge between
Phil Mag., July, 1870.
344-1
RESISTANCE BOXES.
393
two of them as a shunt, the resistance of the corresponding coil
may be put out of the circuit. This arrangement was introduced
by Siemens.
Each interval between the electrodes is marked with the resist
ance of the corresponding coil, so that if we wish to make the
resistance box equal to 107 we express 107 in the binary scale as
64 + 32 + 8+2 + 1 or 1101011. We then take the plugs out
of the holes corresponding to 64, 32, 8, 2 and 1, and leave the
plugs in 16 and 4.
This method, founded on the binary scale, is that in which the
smallest number of separate coils is needed, and it is also that
which can be most readily tested. For if we have another coil
equal to 1 we can test the equality of 1 and l', then that of 1 + 1'
and 2, then that of 1 + !'-{- 2 and 4, and so on.
The only disadvantage of the arrangement is that it requires
a familiarity with the binary scale of notation, which is not
generally possessed by those accustomed to express every number
in the decimal scale.
344.] A box of resistance coils may be arranged in a different
way for the purpose of mea
suring conductivities instead of
resistances.
The coils are placed so that
one end of each is connected
with a long thick piece of
metal which forms one elec
trode of the box, and the other
Fig. 29.
end is connected with a stout piece of brass plate as in the former
case.
The other electrode of the box is a long brass plate, such that
by inserting brass plugs between it and the electrodes of the coils
it may be connected to the first electrode through any given set of
coils. The conductivity of the box is then the sum of the con
ductivities of the coils.
In the figure, in which the resistances of the coils are 1,2,4, &c.,
and the plugs are inserted at 2 and 8, the conductivity of the
box is J+-J = f, and the resistance of the box is therefore f
or 1.6.
This method of combining resistance coils for the measurement
of fractional resistances was introduced by Sir W. Thomson under
the name of the method of multiple arcs. See Art. 276.
394
MEASUREMENT OF EESISTANCE.
[345-
On the Comparison of Resistances.
345.] If E is the electromotive force of a battery, and R the
resistance of the battery and its connexions, including the galvan
ometer used in measuring the current, and if the strength of the
current is / when the battery connexions are closed, and I19 I2
when additional resistances rl3 i\2 are introduced into the circuit,
then, by Ohm's Law,
E=IR = Ii (R + rJ = /2 (R + r2).
Eliminating E, the electromotive force of the battery, and R
the resistance of the battery and its connexions, we get Ohm's
formula _
This method requires a measurement of the ratios of /, /j and 72,
and this implies a galvanometer graduated for absolute mea
surements.
If the resistances ^ and r2 are equal, then /x and 72 are equal,
and we can test the equality of currents by a galvanometer which
is not capable of determining their ratios.
But this is rather to be taken as an example of a faulty method
than as a practical method of determining resistance. The electro
motive force E cannot be maintained rigorously constant, and the
internal resistance of the battery is also exceedingly variable, so
that any methods in which these are assumed to be even for a short
time constant are not to be depended on.
346.] The comparison of resistances can be made with extreme
c
accuracy by either of two methods, in which the result is in
dependent of variations of R and E.
346.] COMPARISON OF RESISTANCES. 395
The first of these methods depends on the use of the differential
galvanometer, an instrument in which there are two coils, the
currents in which are independent of each other, so that when
the currents are made to flow in opposite directions they act in
opposite directions on the needle, and when the ratio of these
currents is that of m to n they have no resultant effect on the
galvanometer needle.
Let Il} T2 be the currents through the two coils of the galvan
ometer, then the deflexion of the needle may be written
Now let the battery current / be divided between the coils of
the galvanometer, and let resistances A and B be introduced into
the first and second coils respectively. Let the remainder of the
resistance of their coils and their connexions be a and (3 respect
ively, and let the resistance of the battery and its connexions
between C and D be ;•, and its electromotive force E.
Then we find, by Ohm's Law, for the difference of potentials
between C and D,
C-D = /jM + a) = L(B + f¥) = E-Ir,
and since /j + 72 — /,
, I=E
where D = (A + a)(£+p)+t(A+d+JB+p).
The deflexion of the galvanometer needle is therefore
5= ~{m(B + p)-u(A+a)},
and if there is no observable deflexion, then we know that the
quantity enclosed in brackets cannot differ from zero by more than
a certain small quantity, depending on the power of the battery,
the suitableness of the arrangement, the delicacy of the galvan
ometer, and the accuracy of the observer.
Suppose that B has been adjusted so that there is no apparent
deflexion.
Now let another conductor A' be substituted for A, and let
A' be adjusted till there is no apparent deflexion. Then evidently
to a first approximation A '= A.
To ascertain the degree of accuracy of this estimate, let the
altered quantities in the second observation be accented, then
396 MEASUREMENT OF EESISTANCE. [346-
m
Hence n (A' -A) = - 5 - 5'.
/^ ^
If 8 and £', instead of being- both apparently zero, had been only
observed to be equal, then, unless we also could assert that E = E',
the right-hand side of the equation might not be zero. In fact,
the method would be a mere modification of that already described.
The merit of the method consists in the fact that the thing
observed is the absence of any deflexion, or in other words, the
method is a Null method, one in which the non-existence of a force
is asserted from an observation in which the force, if it had been
different from zero by more than a certain small amount, would
have produced an observable effect.
Null methods are of great value where they can be employed, but
they can only be employed where we can cause two equal and
opposite quantities of the same kind to enter into the experiment
together.
In the case before us both 8 and b' are quantities too small to be
observed, and therefore any change in the value of E will not affect
the accuracy of the result.
The actual degree of accuracy of this method might be ascer
tained by taking a number of observations in each of which A'
is separately adjusted, and comparing the result of each observation
with the mean of the whole series.
But by putting A' out of adjustment by a known quantity, as,
for instance, by inserting at A or at B an additional resistance
equal to a hundredth part of A or of B, and then observing
the resulting deviation of the galvanometer needle, we can estimate
the number of degrees corresponding to an error of one per cent.
To find the actual degree of precision we must estimate the smallest
deflexion which could not escape observation, and compare it with
the deflexion due to an error of one per cent.
* If the comparison is to be made between A and B, and if the
positions of A and B are exchanged, then the second equation
becomes
* This investigation is taken from Weber's treatise on Galvanometry. Gottingen
Transactions, x. p. 65.
346.] DIFFERENTIAL GALVANOMETER. 397
'
D jy
whence (m + n) (B—A) = -- 5 — -=-, I'.
±j £j
If m and », ^ and B, a and £ are approximately equal, then
B-A =
Here 8 — 8' may be taken to be the smallest observable deflexion
of the galvanometer.
If the galvanometer wire be made longer and thinner, retaining
the same total mass, then n will vary as the length of the wire
and a as the square of the length. Hence there will be a minimum
(A + a)
value of —
a =
If we suppose r, the battery resistance, small compared with A,
this gives a = i ^ .
or, M£ resistance of each coil of the galvanometer should be one-third
of the resistance to be measured.
We then find o ^2
7? / /S *'\
±> — A = 77 (o — o ;.
9 nE v
If we allow the current to flow through one only of the coils
of the galvanometer, and if the deflexion thereby produced is A
(supposing the deflexion strictly proportional to the deflecting
force), then
mE 3 nE .„ _ , 1 ,
A = — - - = - — T if r — 0 and a = - A.
A+a+r ± A 3
B-A 2 5-5r
Hence -^ = - —
In the differential galvanometer two currents are made to
produce equal and opposite effects on the suspended needle. The
force with which either current acts on the needle depends not
only on the strength of the current, but on the position of the
windings of the wire with respect to the needle. Hence, unless
the coil is very carefully wound, the ratio of m to n may change
when the position of the needle is changed, and therefore it is
necessary to determine this ratio by proper methods during each
398 MEASUREMENT OF RESISTANCE. [347-
course of experiments if any alteration of the position of the needle
is suspected.
The other null method, in which Wheatst one's Bridge is used,
requires only an ordinary galvanometer, and the observed zero
deflexion of the needle is due, not to the opposing action of two
currents, but to the non-existence of a current in the wire. Hence
we have not merely a null deflexion, but a null current as the
phenomenon observed, and no errors can arise from want of
regularity or change of any kind in the coils of the galvanometer.
The galvanometer is only required to be sensitive enough to detect
the existence and direction of a current, without in any way
determining its value or comparing its value with that of another
current.
347.] Wheatstone's Bridge consists essentially of six conductors
connecting four points. An electromotive
force E is made to act between two of the
points by means of a voltaic battery in
troduced between B and C. The current
between the other two points 0 and A is
measured by a galvanometer.
Under certain circumstances this current
becomes zero. The conductors BC and OA
are then said to be conjugate to each other,
which implies a certain relation between the resistances of the
other four conductors, and this relation is made use of in measuring
resistances.
If the current in OA is zero, the potential at 0 must be equal
to that at A. Now when we know the potentials at B and C we
can determine those at 0 and A by the rule given at Art. 274,
provided there is no current in OA,
n_By+C(3 A_
/3 + y
whence the condition is fin _ c
where b, c> (3, y are the resistances in CA, AB, BO and OC re
spectively.
To determine the degree of accuracy attainable by this method
we must ascertain the strength of the current in OA when this
condition is not fulfilled exactly.
Let A, B, C and 0 be the four points. Let the currents along
BC, CA and AB be x, y and z, and the resistances of these
348.]
WHEATSTONE'S BRIDGE.
399
conductors a, I and c. Let the currents along OA, OB and OC be
f, j], £ and the resistances a, ft and y. Let an electromotive force
E act along BC. Required the current £ along OA.
Let the potentials at the points A, B, C and 0 be denoted
by the symbols A, B, C and 0. The equations of conduction are
ax=B-C+E, a£ = 0-A,
fy=C-A, prj^O-B,
cz = A-B, y( = 0-C;
with the equations of continuity
£+jr-jf= 0,
Y] + Z — X = 0,
C+x-y = 0.
By considering the system as made up of three circuits OBC,
OCA and OAB in which the currents are x, y, z respectively, and
applying Kirchhoff's rule to each cycle, we eliminate the values
of the potentials 0, A, B, C, and the currents f, r/_, £ and obtain the
following equations for x, y and 0,
— yx
Hence, if we put
-ay
a+ /3 + y
— y
-/3
-y
5 + y + a
— a
-£
— a
e + a + /3
we find
XT
= — (5/3— cy),
and
a? =
Tl
—
348.] The value of D may be expressed in the symmetrical form,
or, since we suppose the battery in the conductor a and the
galvanometer in a, we may put B the battery resistance for a and
G the galvanometer resistance for a. We then find
D =
If the electromotive force E were made to act along OA, the
resistance of OA being still a, and if the galvanometer were placed
400 MEASUREMENT OF RESISTANCE. [349-
in BC) the resistance of BC being still a, then the value of D
would remain the same, and the current in BC due to the electro
motive force E acting along- OA would be equal to the current
in OA due to the electromotive force E acting in BC.
But if we simply disconnect the battery and the galvanometer,
and without altering their respective resistances connect the battery
to 0 and A and the galvanometer to B and C9 then in the value of
D we must exchange the values of B and G. If D' be the value
of D after this exchange, we find
Let us suppose that the resistance of the galvanometer is greater
than that of the battery.
Let us also suppose that in its original position the galvanometer
connects the junction of the two conductors of least resistance /3, y
with the junction of the two conductors of greatest resistance b} c3
or, in other words, we shall suppose that if the quantities />, <?, y, (3
are arranged in order of magnitude, b and c stand together, and
y and /3 stand together. Hence the quantities b — ft and c — y are
of the same sign, so that their product is positive, and therefore
D'— D is of the same sign as B— G.
If therefore the galvanometer is made to connect the junction of
the two greatest resistances with that of the two least, and if
the galvanometer resistance is greater than that of the battery,
then the value of 2) will be less, and the value of the deflexion
of the galvanometer greater, than if the connexions are exchanged.
The rule therefore for obtaining the greatest galvanometer de
flexion in a given system is as follows :
Of the two resistances, that of the battery and that of the
galvanometer, connect the greater resistance so as to join the two
greatest to the two least of the four other resistances.
349.] We shall suppose that we have to determine the ratio of
the resistances of the conductors AB and AC, and that this is to be
done by finding a point 0 on the conductor J30C, such that when
the points A and 0 are connected by a wire, in the course of which
a galvanometer is inserted, no sensible deflexion of the galvano
meter needle occurs when the battery is made to act between B
and C.
The conductor BOC may be supposed to be a wire of uniform
resistance divided into equal parts, so that the ratio of the resist
ances of BO and OC may be read off at once.
349-] WHEATSTONE'S BRIDGE. 401
Instead of the whole conductor being- a uniform wire, we may
make the part near 0 of such a wire, and the parts on each side
may be coils of any form, the resistance of which is accurately
known.
We shall now use a different notation instead of the symmetrical
notation with which we commenced.
Let the whole resistance of BAG be R.
Let c = mE and b = (l—m) R.
Let the whole resistance of BOC be S.
Let /3 = nS and y = (I — n] S.
The value of n is read off directly, and that of m is deduced from
it when there is no sensible deviation of the galvanometer.
Let the resistance of the batteiy and its connexions be B, and
that of the galvanometer and its connexions G.
We find as before
D = G{BR + BS+RS}+m(\-m)R*(B + S} +
+ (**+*— 2m*)£RS,
and if f is the current in the galvanometer wire
t
C =
In order to obtain the most accurate results we must make the
deviation of the needle as great as possible compared with the value
of (n — mj. This may be done by properly choosing the dimensions
of the galvanometer and the standard resistance wire.
It will be shewn, when we come to Galvanometry, Art. 716,
that when the form of a galvanometer wire is changed while
its mass remains constant, the deviation of the needle for unit
current is proportional to the length, but the resistance increases
as the square of the length. Hence the maximum deflexion is
shewn to occur when the resistance of the galvanometer wire is
equal to the constant resistance of the rest of the circuit.
In the present case, if 8 is the deviation,
5 = CV££
where C is some constant, and G is the galvanometer resistance
which varies as the square of the length of the wire. Hence we
find that in the value of D, when 8 is a maximum, the part
involving G must be made equal to the rest of the expression.
If we also put m = «, as is the case if we have made a correct
observation, we find the best value of G to be
G = n(\—n)(R + 8).
VOL. I. D d
402
MEASUREMENT OF RESISTANCE.
[350.
This result is easily obtained by considering the resistance from
A to 0 through the system, remembering that BC, being conjugate
to A0y has no effect on this resistance.
In the same way we should find that if the total area of the
acting surfaces of the battery is given, the most advantageous
arrangement of the battery is when
*=1OT
Finally, we shall determine the value of S such that a given
change in the value of n may produce the greatest galvanometer
deflexion. By differentiating the expression for £ we find
g* -
~
If we have a great many determinations of resistance to make
in which the actual resistance has nearly the same value, then it
may be worth while to prepare a galvanometer and a battery for
this purpose. In this case we find that the best arrangement is
S=R, B=\R, G = 2n(l-n)R,
and if n = i G= \R.
On the Use of Wheatstone's Bridge.
350.] We have already explained the general theory of Wheat-
stone's Bridge, we shall now consider some of its applications.
_LJ -i : I ' ' '; I. M I I I I I
Fig. 32.
The comparison which can be effected with the greatest exact
ness is that of two equal resistances.
35o.] USE OF WHEATSTONE'S BRIDGE. 403
Let us suppose that (3 is a standard resistance coil, and that
we wish to adjust y to be equal in resistance to /3.
Two other coils, b and c, are prepared which are equal or nearly
equal to each other, and the four coils are placed with their electrodes
in mercury cups so that the current of the battery is divided
between two branches, one consisting of /3 and y and the other
of b and c. The coils b and c are connected by a wire P7?, as
uniform in its resistance as possible, and furnished with a scale
of equal parts.
The galvanometer wire connects the junction of /3 and y with
a point Q of the wire PR, and the point of contact at Q is made
to vary till on closing first the battery circuit and then the
galvanometer circuit, no deflexion of the galvanometer needle is
observed.
The coils /3 and y are then made to change places, and a new
position is found for Q. If this new position is the same as the
old one, then we know that the exchange of y3 and y has produced
no change in the proportions of the resistances, and therefore y
is rightly adjusted. If Q has to be moved, the direction and
amount of the change will indicate the nature and amount of the
alteration of the length of the wire of y, which will make its
resistance equal to that of (3.
If the resistances of the coils b and c, each including part of the
wire PJR up to its zero reading, are equal to that of b and c
divisions of the wire respectively, then, if x is the scale reading
of Q in the first case, and y that in the second,
c + z __ _/3_ c+y __ y_
b — x~ y b—y ~~ /3
whence z! = 1
/3-
Since b— y is nearly equal to c + x, and both are great with
respect to x or y, we may write this
and
When y is adjusted as well as we can, we substitute for b and c
other coils of (say) ten times greater resistance.
The remaining difference between /3 and y will now produce
a ten times greater difference in the position of Q than with the
D d 2
404
MEASUREMENT OF RESISTANCE.
original coils b and c, and in this way we can continually increase
the accuracy of the comparison.
The adjustment by means of the wire with sliding contact piece
is more quickly made than by means of a resistance box, and it is
capable of continuous variation.
The battery must never be introduced instead of the galvano
meter into the wire with a sliding contact, for the passage of a
powerful current at the point of contact would injure the surface
of the wire. Hence this arrangement is adapted for the case in
which the resistance of the galvanometer is greater than that of the
battery.
On the Measurement of Small Resistances.
351.] When a short and thick conductor is introduced into a
circuit its resistance is so small compared with the resistance
occasioned by unavoidable faults in the connexions, such as want
of contact or imperfect soldering, that no correct value of the
^ resistance can be deduced from experi
ments made in the way described above.
The object of such experiments is
generally to determine the specific re
sistance of the substance, and it is re
sorted to in cases when the substance
cannot be obtained in the form of a
long thin wire, or when the resistance
to transverse as well as to longitudinal
conduction has to be measured.
C
H
Q
f*
ft
^ y-J
V
Fig. 34.
Sir W. Thomson,* has described a method applicable to such
cases, which we may take as an example of a system of nine
conductors.
* Proc. K. 8., June 6, 1861.
35i.] THOMSON'S METHOD FOR SMALL RESISTANCES. 405
The most important part of the method consists in measuring
the resistance, not of the whole length of the conductor, but of
the part between two marks on the conductor at some little dis
tance from its ends.
The resistance which we wish to measure is that experienced
by a current whose intensity is uniform in any section of the
conductor, and which flows in a direction parallel to its axis.
Now close to the extremities, when the current is introduced
by means of electrodes, either soldered, amalgamated, or simply
pressed to the ends of the conductor, there is generally a want of
uniformity in the distribution of the current in the conductor.
At a short distance from the extremities the current becomes
sensibly uniform. The student may examine for himself the
investigation and the diagrams of Art. 193, where a current is
introduced into a strip of metal with parallel sides through one
of the sides, but soon becomes itself parallel to the sides.
The resistance of the conductors between certain marks S, S'
and TT' is to be compared.
The conductors are placed in series, and with connexions as
perfectly conducting as possible, in a battery circuit of small resist
ance. A wire SVT is made to touch the conductors at S and T,
and S' V'T' is another wire touching them at S' and T.
The galvanometer wire connects the points Fand V of these wires.
The wires SVT and S'V'T' are of resistance so great that the
resistance due to imperfect connexion at S, T, S' or T' may be
neglected in comparison with the resistance of the wire, and F, V
are taken so that the resistance in the branches of either wire
leading to the two conductors are nearly in the ratio of the resist
ances of the two conductors.
Calling H and .Fthe resistances of the conductors SS' and TT.
„ A and C those of the branches SF and FT.
P and R those of the branches S'V and V'T'.
„ Q that of the connecting piece S'T'.
,, B that of the battery and its connexions.
„ G that of the galvanometer and its connexions.
The symmetry of the system may be understood from the
skeleton diagram. Fig. 33.
The condition that B the battery and G the galvanometer may
be conjugate conductors is, in this case,
I. *L (1L A Q
~C " A + \C A'
406
MEASUREMENT OF RESISTANCE.
[352.
Now the resistance of the connector Q is as small as we can
make it. If it were zero this equation would be reduced to
F^_ H
C^-~A'
and the ratio of the resistances of the conductors to be compared
would be that of C to A, as in Wheatstone's Bridge in the ordinary
form.
In the present case the value of Q is small compared with P
or with R, so that if we assume the points V, V so that the ratio
of R to C is nearly equal to that of P to A, the last term of the
equation will vanish, and we shall have
FiHn C-.A.
The success of this method depends in some degree on the per
fection of the contact between the wires and the tested conductors
at SS', T' and T. In the following method, employed by Messrs.
Matthiessen and Hockin *, this condition is dispensed with.
! B
Fig. 35.
352.] The conductors to be tested are arranged in the manner
already described, with the connexions as well made as possible,
and it is required to compare the resistance between the marks
SS' on the first conductor with the resistance between the marks
T' Ton the second.
Two conducting points or sharp edges are fixed in a piece of
insulating material so that the distance between them can be
accurately measured. This apparatus is laid on the conductor to
be tested, and the points of contact with the conductor are then
at a known distance SS'. Each of these contact pieces is connected
* Laboratory. Matthiessen and Hockin on Alloys.
352.] MATTHIESSEN AND HOOKIES METHOD. 407
with a mercury cup, into which one electrode of the galvanometer
may be plunged.
The rest of the apparatus is arranged, as in Wheatstone's Bridge,
with resistance coils or boxes A and (7, and a wire PE with a
sliding contact piece Q, to which the other electrode of the galva
nometer is connected.
Now let the galvanometer be connected to S and Q, and let
Al and Cl be so arranged, and the position of Q so determined, that
there is no current in the galvanometer wire.
Then we know that ^ A -\-PQ
W'= cl+QR
where XS, PQ, &c. stand for the resistances in these conductors.
From this we get
XS A
Now let the electrode of the galvanometer be connected to S't
and let resistance be transferred from C to A (by carrying resistance
coils from one side to the other) till electric equilibrium of the
galvanometer wire can be obtained by placing Q at some point
of the wire, say Q2. Let the values of C and A be now <?2 and A2,
and let A2+C.2 + PR = A^ + C^ + PR = R.
Then we have, as before,
XS' _A2+PQ2
XT' R
SS' Ay-Ai+QiQt
Whence jy = - -g-
In the same way, placing the apparatus on the second conductor
at TT' and again transferring resistance, we get, when the electrode
is in T',
XT'
XT' R
and when it is in T,
XT
XT' R
Whence
AI K
We can now deduce the ratio of the resistances SS' and T'T, for
SS* _A2-A1+Q1Q2
TT A,-A3+Q,Q,'
*
408 MEASUREMENT OF RESISTANCE. [353-
When great accuracy is not required we may dispense with the
resistance coils A and C, and we then find
SS' _ Q, Q2
?"T~ Q3Q*
The readings of the position of Q on a wire of a metre in length
cannot be depended on to less than a tenth of a millimetre, and the
resistance of the wire may vary considerably in different parts
owing to inequality of temperature, friction, &c. Hence, when
great accuracy is required, coils of considerable resistance are intro
duced at A and (?, and the ratios of the resistances of these coils
can be determined more accurately than the ratio of the resistances
of the parts into which the wire is divided at Q.
It will be observed that in this method the accuracy of the
determination depends in no degree on the perfection of the con
tacts at SS' or TT'.
This method may be called the differential method of using
Wheatstone's Bridge, since it depends on the comparison of ob
servations separately made.
An essential condition of accuracy in this method is that the
resistance of the connexions should continue the same during the
course of the four observations required to complete the determ
ination. Hence the series of observations ought always to be
repeated in order to detect any change in the resistances.
On the Comparison of Great Resistances.
353.] When the resistances to be measured are very great, the
comparison of the potentials at different points of the system may
be made by means of a delicate electrometer, such as the Quadrant
Electrometer described in Art. 219.
If the conductors whose resistance is to be measured are placed
in series, and the same current passed through them by means of a
battery of great electromotive force, the difference of the potentials
at the extremities of each conductor will be proportional to the
resistance of that conductor. Hence, by connecting the electrodes
of the electrometer with the extremities, first of one conductor
and then of the other, the ratio of their resistances may be de
termined.
This is the most direct method of determining resistances. It
involves the use of an electrometer whose readings may be depended
on, and we must also have some guarantee that the current remains
constant during the experiment.
355-] GEEAT RESISTANCES. 409
Four conductors of great resistance may also be arranged as in
Wheatstone's Bridge, and the bridge itself may consist of the
electrodes of an electrometer instead of those of a galvanometer.
The advantage of this method is that no permanent current is
required to produce the deviation of the electrometer, whereas the
galvanometer cannot be deflected unless a current passes through
the wire.
354.] When the resistance of a conductor is so great that the
current which can be sent through it by any available electromotive
force is too small to be directly measured by a galvanometer, a
condenser may be used in order to accumulate the electricity for
a certain time, and then, by discharging the condenser through a
galvanometer, the quantity accumulated may be estimated. This
is Messrs. Bright and Clark's method of testing the joints of
submarine cables.
355.] But the simplest method of measuring the resistance of
such a conductor is to charge a condenser of great capacity and to
connect its two surfaces with the electrodes of an electrometer
and also with the extremities of the conductor. If E is the dif
ference of potentials as shewn by the electrometer, S the capacity
of the condenser, and Q the charge on either surface, R the resist
ance of the conductor and x the current in it, then, by the theory
of condensers, n — ?&
By Ohm's Law, E = Ex,
and by the definition of a current,
Hence
and Q=QQe~™y
where Q0 is the charge at first when t = 0 .
t
Similarly E = E0e~**
where E0 is the original reading of the electrometer, and E the
same after a time t. From this we find
-R- —
~S{logA-loge#}'
which gives R in absolute measure. In this expression a knowledge
of the value of the unit of the electrometer scale is not required.
410
MEASUREMENT OF RESISTANCE.
[356-
If S, the capacity of the condenser, is given in electrostatic
measure as a certain number of metres, then R is also given in
electrostatic measure as the reciprocal of a velocity.
If S is given in electromagnetic measure its dimensions are
%>2
- - , and R is a velocity.
Jj
Since the condenser itself is not a perfect insulator it is necessary
to make two experiments. In the first we determine the resistance
of the condenser itself, 2t0, and in the second, that of the condenser
when the conductor is made to connect its surfaces. Let this be Rf.
Then the resistance, R, of the conductor is given by the equation
1 1 1
R R' RQ
This method has been employed by MM. Siemens.
Thomsons * Method for the Determination of the Resistance of
the Galvanometer.
356.] An arrangement similar to Wheatstone's Bridge has been
employed with advantage by Sir W. Thomson in determining the
Fig. 36.
resistance of the galvanometer when in actual use. It was sug
gested to Sir W. Thomson by Mance's Method. See Art. 357.
Let the battery be placed, as before, between £ and C in the
figure of Article 347, but let the galvanometer be placed in CA
instead of in OA. If bfi — cy is zero, then the conductor OA is
conjugate to J3C, and, as there is no current produced in 0 A by the
battery in JBC, the strength of the current in any other conductor
* Proc. R. S,, Jan. 19, 1871.
357-] MANCE'S METHOD. 411
is independent of the resistance in OA. Hence, if the galvano
meter is placed in CA its deflexion will remain the same whether
the resistance of OA is small or great. We therefore observe
whether the deflexion of the galvanometer remains the same when
0 and A are joined by a conductor of small resistance, as when
this connexion is broken, and if, by properly adjusting the re
sistances of the conductors, we obtain this result, we know that
the resistance of the galvanometer is
.
where c, y, and /3 are resistance coils of known resistance.
It will be observed that though this is not a null method, in the
sense of there being no current in the galvanometer, it is so in
the sense of the fact observed being the negative one, that the
deflexion of the galvanometer is not changed when a certain con
tact is made. An observation of this kind is of greater value
than an observation of the equality of two different deflexions of
the same galvanometer, for in the latter case there is time for
alteration in the strength of the battery or the sensitiveness of
the galvanometer, whereas when the deflexion remains constant,
in spite of certain changes which we can repeat at pleasure, we are
sure that the current is quite independent of these changes.
The determination of the resistance of the coil of a galvanometer
can easily be effected in the ordinary way of using Wheatstone's
Bridge by placing another galvanometer in OA. By the method
now described the galvanometer itself is employed to measure its
own resistance.
nance's* Method of determining the Resistance of the Battery.
357.] The measurement of the resistance of a battery when in
action is of a much higher order of difficulty, since the resistance
of the battery is found to change considerably for some time after
the strength of the current through it is changed. In many of the
methods commonly used to measure the resistance of a battery such
alterations of the strength of the current through it occur in the
course of the operations, and therefore the results are rendered
doubtful.
In Mance's method, which is free from this objection, the battery
is placed in BC and the galvanometer in CA. The connexion
between 0 and B is then alternately made and broken.
* Proc. R. S., Jan. 19, 1871.
412 MEASUREMENT OF RESISTANCE. [357-
If the deflexion of the galvanometer remains unaltered, we know
that OB is conjugate to CA, whence cy = a a, and a, the resistance
of the battery, is obtained in terms of known resistances c, y, a.
When the condition cy = a a is fulfilled, then the current through
the galvanometer is
Ea
and this is independent of the resistance (3 between 0 and B. To
test the sensibility of the method let us suppose that the condition
cy = aa is nearly, but not accurately, fulfilled, and that y0 is the
Fig. 37.
current through the galvanometer when 0 and B are connected
by a conductor of no sensible resistance, and y± the current when
0 and B are completely disconnected.
To find these values we must make /3 equal to 0 and to oo in the
general formula for y, and compare the results.
In this way we find
y*-y\ _ <* cy—aa
where y§ an(i y\ are supposed to be so nearly equal that we may,
when their difference is not in question, put either of them equal
to y, the value of the current when the adjustment is perfect.
The resistance, c, of the conductor AB should be equal to a,
that of the battery, a and y, should be equal and as small as
possible, and b should be equal to a + y.
Since a galvanometer is most sensitive when its deflexion is
small, we should bring the needle nearly to zero by means of fixed
magnets before making contact between 0 and B.
In this method of measuring the resistance of the battery, the
current in the battery is not in any way interfered with during the
operation, so that we may ascertain its resistance for any given
358.]
COMPARISON OF ELECTROMOTIVE FORCES.
413
strength of current, so as to determine how the strength of current
effects the resistance.
If y is the current in the galvanometer, the actual current
through the battery is #0 with the key down and x: with the
key up, where
/ b \ f I ac x
*o = y(1 + -r-)' *i = y(l +- + -T
> a + y y vifl1
the resistance of the battery is
cy
a = — - ,
a
and the electromotive force of the battery is
The method of Art. 356 for finding the resistance of the galva
nometer differs from this only in making and breaking contact
between 0 and A instead of between 0 and .Z?, and by exchanging
a and 3 we obtain for this case
On the Comparison of Electromotive Forces.
358.] The following method of comparing the electromotive forces
of voltaic and thermoelectric arrangements, when no current passes
through them, requires only a set of resistance coils and a constant
battery.
Let the electromotive force E of the battery be greater than that
of either of the electromotors to be compared, then, if a sufficient
iHHHI-
E
Fig. 38.
resistance, S19 be interposed between the points Alt Sl of the
primary circuit E Bl Al E, the electromotive force from A to A1
414
MEASUREMENT OF RESISTANCE.
[358.
may be made equal to that of the electromotor E^. If the elec
trodes of this electromotor are now connected with the points
A19 B± no current will flow through the electromotor. By placing
a galvanometer Gl in the circuit of the electromotor E^9 and
adjusting the resistance between Al and 12l9 till the galvanometer
G} indicates no current, we obtain the equation
where Rl is the resistance between Al and S19 and C is the strength
of the current in the primary circuit.
In the same way, by taking a second electromotor E2 and placing
its electrodes at A.2 and JB2, so that no current is indicated by the
galvanometer G2,
E2 = Z22<7,
where 7?2 is the resistance between A.2 and B2. If the observations
of the galvanometers Gl and G2 are simultaneous, the value of C,
the current in the primary circuit, is the same in both equations,
and we find
E, : ^ : : Kl : St.
In this way the electromotive force of two electromotors may be
compared. The absolute electromotive force of an electromotor
may be measured either electrostatically by means of the electro
meter, or electromagnetically by means of an absolute galvano
meter.
This method, in which, at the time of the comparison, there
is no current through either of the electromotors, is a modification
of Poggendorff 's method, and is due to Mr. Latimer Clark, who
has deduced the following values of electromotive forces :
Daniell I. Amalgamated Zinc HS04 + 4 aq.
II.
III.
Bunsen I.
II.
Grove
HS04+12aq.
HS04 + 12aq.
HS04+ 4aq.
Concentrated
V It
solution of
o s.
CuSO4
Copper
= 1.079
CuS04
Copper
= 0.978
CuN06
Copper
= 1.00
HN06
Carbon
= 1.964
sp. g. 1.38
Carbon
= 1.888
HN06
Platinum
= 1.956
A Volt is an electromotive force equal to 100,000,000 units of the centimetre-gramme-
second system.
CHAPTER XII.
ON THE ELECTRIC RESISTANCE OF SUBSTANCES.
359.] THERE are three classes in which we may place different
substances in relation to the passage of electricity through them.
The first class contains all the metals and their alloys, some
sulphurets, and other compounds containing metals, to which we
must add carbon in the form of gas-coke, and selenium in the
crystalline form.
In all these substances conduction takes place without any
decomposition, or alteration of the chemical nature of the substance,
either in its interior or where the current enters and leaves the
body. In all of them the resistance increases as the temperature
rises.
The second class consists of substances which are called electro
lytes, because the current is associated with a decomposition of
the substance into two components which appear at the electrodes.
As a rule a substance is an electrolyte only when in the liquid
form, though certain colloid substances, such as glass at 100°C,
which are apparently solid, are electrolytes. It would appear from
the experiments of Sir B. C. Brodie that certain gases are capable
of electrolysis by a powerful electromotive force.
In all substances which conduct by electrolysis the resistance
diminishes as the temperature rises.
The third class consists of substances the resistance of which is
so great that it is only by the most refined methods that the
passage of electricity through them can be detected. These are
called Dielectrics. To this class belong a considerable number
of solid bodies, many of which are electrolytes when melted, some
liquids, such as turpentine, naphtha, melted paraffin, &c., and all
gases and vapours. Carbon in the form of diamond, and selenium
in the amorphous form, belong to this class.
The resistance of this class of bodies is enormous compared with
that of the metals. It diminishes as the temperature rises. It
416 RESISTANCE. [360.
is difficult, on account of the great resistance of these substances,
to determine whether the feeble current which we can force through
them is or is not associated with electrolysis.
On the Electric Resistance of Metals.
360.] There is no part of electrical research in which more
numerous or more accurate experiments have been made than in
the determination of the resistance of metals. It is of the utmost
importance in the electric telegraph that the metal of which the
wires are made should have the smallest attainable resistance.
Measurements of resistance must therefore be made before selecting
the materials. When any fault occurs in the line, its position is
at once ascertained by measurements of resistance, and these mea
surements, in which so many persons are now employed, require
the use of resistance coils, made of metal the electrical properties
of which have been carefully tested.
The electrical properties of metals and their alloys have been
studied with great care by MM. Matthiessen, Vogt, and Hockin,
and by MM. Siemens, who have done so much to introduce exact
electrical measurements into practical work.
It appears from the researches of Dr. Matthiessen, that the effect
of temperature on the resistance is nearly the same for a considerable
number of the pure metals, the resistance at 100°C being to that
at 0°C in the ratio of 1.414 to 1, or of 1 to 70.7. For pure iron
the ratio is 1.645, and for pure thallium 1.458.
The resistance of metals has been observed by Dr. C.W. Siemens*"
through a much wider range of temperature, extending from the
freezing point to 350°C, and in certain cases to 1000°C. He finds
that the resistance increases as the temperature rises, but that the
rate of increase diminishes as the temperature rises. The formula,
which he finds to agree very closely both with the resistances
observed at low temperatures by Dr. Matthiessen and with his
own observations through a range of 1000°C, is
where T is the absolute temperature reckoned from — 273°C, and
a, /3, y are constants. Thus, for
Platinum ...... r = 0.0393697^+ 0.0021640777— 0.2413,
Copper ......... r = 0.0265777^+0. 0031443^—0.22751,
Iron ............ r= 0. 0725457^ + 0.0038 13377— 1.23971.
* Proc. R. S., April 27, 1871.
361.] OF METALS. 417
From data of this kind the temperature of a furnace may be
determined by means of an observation of the resistance of a
platinum wire placed in the furnace.
Dr. Matthiessen found that when two metals are combined to
form an alloy, the resistance of the alloy is in most cases greater
than that calculated from the resistance of the component metals
and their proportions. In the case of alloys of gold and silver, the
resistance of the alloy is greater than that of either pure gold or
pure silver, and, within certain limiting proportions of the con
stituents, it varies very little with a slight alteration of the pro
portions. For this reason Dr. Matthiessen recommended an alloy
of two parts by weight of gold and one of silver as a material
for reproducing the unit of resistance.
The effect of change of temperature on electric resistance is
generally less in alloys than in pure metals.
Hence ordinary resistance coils are made of German silver, on
account of its great resistance and its small variation with tem
perature.
An alloy of silver and platinum is also used for standard coils.
361.] The electric resistance of some metals changes when the
metal is annealed ; and until a wire has been tested by being
repeatedly raised to a high temperature without permanently
altering its resistance, it cannot be relied on as a measure of
resistance. Some wires alter in resistance in course of time without
having been exposed to changes of temperature. Hence it is
important to ascertain the specific resistance of mercury, a metal
which being fluid has always the same molecular structure, and
which can be easily purified by distillation and treatment with
nitric acid. Great care has been bestowed in determining the
resistance of this metal by W. and C. F. Siemens, who introduced
it as a standard. Their researches have been supplemented by
those of Matthiessen and Hockin.
The specific resistance of mercury was deduced from the observed
resistance of a tube of length /'containing a weight 10 of mercury,
in the following manner.
No glass tube is of exactly equal bore throughout, but if a small
quantity of mercury is introduced into the tube and occupies a
length A. of the tube, the middle point of which is distant x from
one end of the tube, then the area s of the section near this point
C
will be s = — , where C is some constant.
A
VOL. I. E 6
418
RESISTANCE.
[362.
= p I s dx =
The weight of mercury which fills the whole tube is
— >
where n is the number of points, at equal distances along- the
tube, where A has been measured, and p is the mass of unit of
volume.
The resistance of the whole tube is
(
J
_ / • 7^ Trli-
— I — UtJ, — ~rT •• l/V ~~
J * 6 «
where r is the specific resistance per unit of volume.
Hence wR =
wR
and
gives the specific resistance of unit of volume.
To find the resistance of unit of length and unit of mass we must
multiply this by the density.
It appears from the experiments of Matthiessen and Hockin that
the resistance of a uniform column of mercury of one metre in
length, and weighing one gramme at 0°C, is 13.071 Ohms, whence
it follows that if the specific gravity of mercury is 13.595, the
resistance of a column of one metre in length and one square
millimetre in section is 0.96146 Ohms.
362.] In the following table R is the resistance in Ohms of a
column one metre long and one gramme weight at 0°C, and r is
the resistance in centimetres per second of a cube of one centi
metre, according to the experiments of Matthiessen *.
Silver
Specific
gravity
10.50
hard drawn
0
R
.1689
r
1609
Percentage
increment of
resistance for
1°C at 20°C.
0.377
Copper .....
Gold
. 8.95
19 27
hard drawn
hard drawn
0
o
.1469
.4150
1642
2154
0.388
0.365
Lead . ...
11.391
pressed
2
.257
19847
0.387
Mercury ,
13.595
liquid 1
3
.071
96146
0.072
Gold 2, Silver 1
Selenium at 100
..15.218
°C
hard or annealed
Crystalline form
1
.668
10988
6xl013
0.065
1.00
Phil. Mag., May, 1865.
364.] OF ELECTROLYTES. 419
On the Electric Resistance of Electrolytes.
363.] The measurement of the electric resistance of electrolytes
is rendered difficult on account of the polarization of the electrodes,
which causes the observed difference of potentials of the metallic
electrodes to be greater than the electromotive force which actually
produces the current.
This difficulty can be overcome in various ways. In certain
cases we can get rid of polarization by using electrodes of proper
material, as, for instance, zinc electrodes in a solution of sulphate
of zinc. By making the surface of the electrodes very large com
pared with the section of the part of the electrolyte whose resist
ance is to be measured, and by using only currents of short duration
in opposite directions alternately, we can make the measurements
before any considerable intensity of polarization has been excited
by the passage of the current.
Finally, by making two different experiments, in one of which
the path of the current through the electrolyte is much longer than
in the other, and so adjusting the electromotive force that the
actual current, and the time during which it flows, are nearly the
same in each case, we can eliminate the effect of polarization
altogether.
364.] In the experiments of Dr. Paalzow * the electrodes were
in the form of large disks placed in separate flat vessels filled with
the electrolyte, and the connexion was made by means of a long
siphon filled with the electrolyte and dipping into both vessels.
Two such siphons of different lengths were used.
The observed resistances of the electrolyte in these siphons
being 7^ and Z?2, the siphons were next filled with mercury, and
their resistances when filled with mercury were found to be R^
and S2'.
The ratio of the resistance of the electrolyte to that of a mass
of mercury at 0°C of the same form was then found from the
formula r> r>
M^ — ti.2
P T> / T> f '
j»j -M2
To deduce from the values of p the resistance of a centimetre in
length having a section of a square centimetre, we must multiply
them by the value of r for mercury at 0°C. See Art. 361.
* Berlin MonatsbericU, July, 1868.
E e 2
420 RESISTANCE.
The results given by Paalzow are as follow : —
Mixtures of Sulphuric Acid and Water.
nr.
with
Resistance compared
mercury.
H2SO4 ____ 15°C 96950
H2SO4 + 14H20 .... 19°C 14157
H2SO4+ 13H2O .... 22°C 13310
H2 SO4 + 499 H2O ____ 22°C 184773
Sulphate of Zinc and Wat&)\
ZnS04 + 23H2O .... 23°C 194400
ZnS04 + 24H20 ____ 23°C 191000
ZnSO4 +105H2O .... 23°C 354000
Sulphate of Copper and Water.
CuSO4+ 45H2O .... 22°C 202410
CuSO4+105H2O .... 22°C 339341
Sulphate of Magnesium and Water.
MgS04+ 34H2O .... 22°C 199180
MgS04+107H20 .... 22°C 324600
Hydrochloric Acid and Water.
HC1 + 15H20 .... 23°C 13626
HC1 + 500H2O ____ 23°C 86679
365.] MM. F. Kohlrausch and W. A. Nippoldt* have de
termined the resistance of mixtures of sulphuric acid and water.
They used alternating1 magneto-electric currents, the electromotive
force of which varied from ^ to T\- of that of a Grove's cell, and
by means of a thermoelectric copper-iron pair they reduced the
electromotive force to 4 a ^0 0 0 of that of a Grove's cell. They found
that Ohm's law was applicable to this electrolyte throughout the
range of these electromotive forces.
The resistance is a minimum in a mixture containing about one-
third of sulphuric acid.
The resistance of electrolytes diminishes as the temperature
increases. The percentage increment of conductivity for a rise of
1°C is given in the following table.
* Pogg., Ann. cxxxviii, p. 286, Oct. 1869.
;66.]
OF ELECTROLYTES.
421
Resistance of Mixtures of Sulphuric Acid and Water at 22°C in terms
of Mercury at 0°C. MM. Kohlrauscli and Nippoldt.
Specific gravity
at 18°5
0.9985
1.00
1.0504
1.0989
1.1431
1.2045
1.2631
1.3163
1.3547
1.3994
1.4482
1.5026
Percentage
of H2 SO,
0.0
0.2
8.3
14.2
20.2
28.0
35.2
41.5
46.0
50.4
55.2
60.3
Resistance
at 227C
746300
465100
34530
18946
14990
13133
13132
14286
15762
17726
20796
25574
Percentage
increment of
conductivity
for 1°C.
0.47
0.47
0.653
0.646
0.799
1.317
1.259
.410
.674
.582
.417
.794
On the Electrical Resistance of Dielectrics.
366.] A great number of determinations of the resistance of
gutta-percha, and other materials used as insulating- media, in the
manufacture of telegraphic cables, have been, made in order to
ascertain the value of these materials as insulators.
The tests are generally applied to the material after it has been
used to cover the conducting wire, the wire being used as one
electrode, and the water of a tank, in which the cable is plunged,
as the other. Thus the current is made to pass through a cylin
drical coating of the insulator of great area and small thickness.
It is found that when the electromotive force begins to act, the
current, as indicated by the galvanometer, is by no means constant.
The first effect is of course a transient current of considerable
intensity, the total quantity of electricity being that required to
charge the surfaces of the insulator with the superficial distribution
of electricity corresponding to the electromotive force. This first
current therefore is a measure not of the conductivity, but of the
capacity of the insulating layer.
But even after this current has been allowed to subside the
residual current is not constant, and does not indicate the true
conductivity of the substance. It is found that the current con
tinues to decrease for at least half an hour, so that a determination
422 RESISTANCE. [366.
of the resistance deduced from the current will give a greater value
if a certain time is allowed to elapse than if taken immediately after
applying the battery.
Thus, with Hooper's insulating material the apparent resistance
at the end of ten minutes was four times, and at the end of
nineteen hours twenty-three times that observed at the end of
one minute. When the direction of the electromotive force is
reversed, the resistance falls as low or lower than at first and then
gradually rises.
These phenomena seem to be due to a condition of the gutta-
percha, which, for want of a better name, we may call polarization,
and which we may compare on the one hand with that of a series
of Leyden jars charged by cascade, and, on the other, with Ritter's
secondary pile, Art. 271.
If a number of Leyden jars of great capacity are connected in
series by means of conductors of great resistance (such as wet
cotton threads in the experiments of M. Gaugain), then an electro
motive force acting on the series will produce a current, as indicated
by a galvanometer, which will gradually diminish till the jars are
fully charged.
The apparent resistance of such a series will increase, and if the
dielectric of the jars is a perfect insulator it will increase without
limit. If the electromotive force be removed and connexion made
between the ends of the series, a reverse current will be observed,
the total quantity of which, in the case of perfect insulation, will be
the same as that of the direct current. Similar effects are observed
in the case of the secondary pile, with the difference that the final
insulation is not so good, and that the capacity per unit of surface
is immensely greater.
In the case of the cable covered with gutta-percha, &c., it is found
that after applying the battery for half an hour, and then con
necting the wire with the external electrode, a reverse current takes
place, which goes on for some time, and gradually reduces the
system to its original state.
These phenomena are of the same kind with those indicated
by the ' residual discharge' of the Leyden jar, except that the
amount of the polarization is much greater in gutta-percha, &c.
than in glass.
This state of polarization seems to be a directed property of the
material, which requires for its production not only electromotive
force, but the passage, by displacement or otherwise, of a con-
368.] OF DIELECTRICS. 423
siderable quantity of electricity, and this passage requires a con
siderable time. When the polarized state has been set up, there
is an internal electromotive force acting1 in the substance in the
reverse direction, which will continue till it has either produced
a reversed current equal in total quantity to the first, or till the
state of polarization has quietly subsided by means of true con
duction through the substance.
The whole theory of what has been called residual discharge,
absorption of electricity, electrification, or polarization,, deserves
a careful investigation, and will probably lead to important dis
coveries relating to the internal structure of bodies.
367.] The resistance of the greater number of dielectrics di
minishes as the temperature rises.
Thus the resistance of gutta-percha is about twenty times as great
at 0°C as at 24°C. Messrs. Bright and Clark have found that the
following formula gives results agreeing with their experiments.
If r is* the resistance of gutta-percha at temperature T centigrade,
then the resistance at temperature T+ 1 will be
PC = r x 0.8878',
the number varies between 0.8878 and 0.9.
Mr. Hockin has verified the curious fact that it is not until some
hours after the gutta-percha has taken its temperature that the
resistance reaches its corresponding value.
The effect of temperature on the resistance of india-rubber is not
so great as on that of gutta-percha.
The resistance of gutta-percha increases considerably on the
application of pressure.
The resistance, in Ohms, of a cubic metre of various specimens of
gutta-percha used in different cables is as follows *.
Name of Cable.
Red Sea .267 x 1012 to .362 x 1012
Malta- Alexandria 1 .23 x 1 012
Persian Gulf 1.80 x 1012
Second Atlantic 3.42 x 1012
Hooper's Persian Gulf Core... 7 4. 7 x 1012
Gutta-percha at 2 4°C 3.53 x 1012
368.] The following table, calculated from the experiments of
* Jenkin's Cantor Lectures.
424 KESISTANCE. [369.
M. Buff, described in Art. 271, shews the resistance of a cubic
metre of glass in Ohms at different temperatures.
Temperature. Resistance.
200°C 227000
250° 13900
300° 1480
350° 1035
400° 735
369.] Mr. C. F. Varley * has recently investigated the conditions
of the current through rarefied gases, and finds that the electro
motive force E is equal to a constant EQ together with a part
depending on the current according to Ohm's Law, thus
For instance, the electromotive force required to cause the
current to begin in a certain tube was that of 323 Dani ell's cells,
but an electromotive force of 304 cells was just sufficient to
maintain the current. The intensity of the current, as measured
by the galvanometer, was proportional to the number of cells above
304. Thus for 305 cells the deflexion was 2, for 306 it was 4,
for 307 it was 6, and so on up to 380, or 304 + 76 for which the
deflexion was 150, or 76 x 1.97.
From these experiments it appears that there is a kind of
polarization of the electrodes, the electromotive force of which
is equal to that of 304 DanielFs cells, and that up to this electro
motive force the battery is occupied in establishing this state of
polarization. When the maximum polarization is established, the
excess of electromotive force above that of 304 cells is devoted to
maintaining the current according to Ohm's Law.
The law of the current in a rarefied gas is therefore very similar
to the law of the current through an electrolyte in which we have
to take account of the polarization of the electrodes.
In connexion with this subject we should study Thomson's results,
described in Art. 57, in which the electromotive force required
to produce a spark in air was found to be proportional not to the
distance, but to the distance together with a constant quantity.
The electromotive force corresponding to this constant quantity
may be regarded as the intensity of polarization of the electrodes.
370.] MM. Wiedemann and Ruhlmann have recently f investi-
* Proc. E. S., Jan. 12, 1871.
f Serichte der Konigl. Sachs. Gesellschafl, Oct. 20, 1871.
370.] OF DIELECTRICS. 425
gated the passage of electricity through gases. The electric current
was produced by Holtz's machine, and the discharge took place
between spherical electrodes within a metallic vessel containing
rarefied gas. The discharge was in general discontinuous, and the
interval of time between successive discharges was measured by
means of a mirror revolving along with the axis of Holtz's machine.
The images of the series of discharges were observed by means of
a heliometer with a divided object-glass, which was adjusted till
one image of each discharge coincided with the other image of
the next discharge. By this method very consistent results were
obtained. It was found that the quantity of electricity in each
discharge is independent of the strength of the current and of
the material of the electrodes, and that it depends on the nature
and density of the gas, and on the distance and form of the
electrodes.
These researches confirm the statement of Faraday* that the
electric tension (see Art. 48) required to cause a disruptive discharge
to begin at the electrified surface of a conductor is a little less
when the electrification is negative than when it is positive, but
that when a discharge does take place, much more electricity passes
at each discharge when it begins at a positive surface. They also
tend to support the hypothesis stated in Art. 57, that the stratum
of gas condensed on the surface of the electrode plays an important
part in the phenomenon, and they indicate that this condensation
is greatest at the positive electrode.
* Exp. Res., 1501.
VOL. i.
F1L
T
VOL . I .
Vol.1.
FIG. I .
Art. 118
of F
orce
Surfaces .
A = ZO . B = 5 . f, . font of fyuiti&ruim .
ofite darmdcn fress .
riG. n.
Art . 119
Lines of Force arid
<z£ Surfaces .
A ^2O. B=-5 F, Point
Q, Spherical wtrfaoe of Zero pntcnti
Af, Ibi,nt of Mnucimujns force along the
The dotted, fane is the Lirte of force Y = O.I
AP = 2 All
For -i/i&. Delegates yflk» Clarmdcn Tress
FIG Hi
Art. 120
Lines
A =
.for ikeDeliyafes oftfe Clarendon, P
fee J%ajr*0H,s JttecirL
Vol.L
FIG. iv.
Art. 121.
Lines of Force and -Ay
-.. • • • • .
f force a.nf^ j£yuijyole/i£t'.<i£ Surfaces fsi- a,
vfci spherical Surface m w^/^yi t/te siy>erficia/ density
is ft harmonic o/ f/ie first
.For tkeDelegates of ike, ClarmdvTiFress.
FIG. vi
Art. J43
Spherical Harmante of the fAmi degree.
= 3 .
jFbr i/ieDelepa£es oflfe Clarendon fress
FIG. VR
Art. 143
of the fJii7-<L deqree.
— 3 .
fbrlk&Delepafes of the Clarmdvn-fress .
I Maxwell's Il*ctricit)r. Vo2 f
FJG. VHI
Art. 143
Spkeristil Harmonic of the /ourttv degree,
i = 4, s = 2 .
FIG. JX
Art. 143
Spherical Jfarmvnic of the fourth, d&jree.
';'
.. Vol.F.
x.
Arc. 192.
E72ip,se.9
(Zarerufaifress
Ma
- • VolL
FIG XL.
Art. 193.
of* Jforce-
the,
&/ *
Limes of* J*orce between two Plates .
Maucwetl's Jllectncity. . Vcl. 2.
FIG-, xm
Art 203
r
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